First I must give credit to queza7 from livejournal (her costume commission site is here). She posted her pattern and instructions here, I simply followed them. Please think of my instructable as a companion piece, not a replacement to her instructions and patterns. Another good set of resources are waywardbound's pictures of a production jacket on flickr.

This is my first intsructables and my first sewing project, so I'm sure that I'll miss something (already took too few pictures) or do something backwards or in a round about way.

Step 1: What You Will Need

Materials

For the Pattern
-Butcher paper for the pattern
-Simplicity pattern 5386

For the Jacket
-About 4.5 yards Cotton twill
- One 125 yard roll of heavy duty white spun polyester thread - I used this for seam construction of for the detail top stitching (you may want matching color thread for your seam construction)
-All purpose thread - used for buttonholes, and finishing cut edges
-9 buttons - I got my buttons off of ebay, I used 8 matching buttons and a slightly smaller button for the collar
-Fusable interfacing- Optional - Helps to stiffen and reinforce fabric, I used it only where my buttonholes were cut

Tools
-Chalk or a bar of soap for pattern tracing - I like soap since it does not wipe away as easily as chalk, and washes out cleanly
-Fine point Sharpie - I used this for alignment marks on the fabric
-lots of pins
-Iron
-Glass table
-Reading lamp
-Sewing Machine, basic model with straight and zigzag stitch
-Buttonholer
-Scissors
-Sharp utility knife

Step 2: Creating the Pattern Part 1

I did not want to cut the original pattern sheets since I was a bit unsure about sizing and was worried about tearing the thin pattern paper. So I used a large glass table to trace the pattern onto butcher paper.

Under the table I used two lights. For the light without the adjustable neck I used a mug to hold the lamp up.

Be sure to copy all the alignment notches and arrows. The waistline mark was also helpful in lining up different pieces.

Back
Use piece 3 and 4 from the simplicity pattern.
Begin by tracing piece 3 at the top of your paper. Using the center line and the alignment notches add piece 4. You can ignore anything that extends past the center line.

I flipped the original simplicity pattern over, lined everything up along the center line and traced the opposite side. You can also fold the paper in half along the center line and cut out the 2 layers giving you the full pattern.

Step 3: Creating the Pattern Part 2

inside front facing
Trace piece 2 from the simplicity pattern. I did not add piece 2A or 1A when tracing, It was easy enough to extrapolate the line out on the outside edge, since it is no longer curving at that point. Add Piece 1 overlapping 5/8 inch, absorbing the seam allowance. I made sure the the 5/8 overlap was correct at the shoulder and at the waist, it varied a bit elsewhere.

outer front facing
For the outer front facing double the pattern over the center line, just like with the back piece.

Step 4: The Rest of the Pattern

Collar
-Trace piece 5 from the simplicity pattern.
-Draw a line perpendicular to the top and bottom edges 1+5/8 inches to right of the circle next to the 5 on the pattern.
-flip simplicity pattern and trance the mirror image, lining up using center fold line.
-flip simplicity patter over again, line up using the circles on the right side of the pattern, smoothly continuing the curve of the first two pieces.
-the final pattern is piece is cut from the left red line to right red line.

Sleeve
Basically unmodified from the Simplicity Pattern. I'm tall, and the original simplicity pattern had a separate cuff piece, so I added 2 inches to the length of the sleeves. I'm not sure if I actually used the added length but better for the sleeves to be too long than too short.

Step 5: Cut Fabric

you probably want to wash and dry your fabric to preshrink it and wash out any excess dye in the fabric. I did not do this, so after washing the finished jacket it shrank a little and my white stitching picked up a faint red hue.

Layout paper patterns on the fabric to minimize waste. You want to maintain the fabric grain throughout the piece. The top to bottom center line of the coat should be parallel to the selvedge of the fabric as they come off the roll.

selvedge refers to the edge of fabric as it comes off the bolt. The selvedge is the edges of the fabric which may manufacturer information. This area of the fabric is usually a bound edge that does not fray.

I traced on the wrong (inside) face of the fabric. I used a bar of dove soap to trace the pattern.

Use a few pins to pin patten to the fabric.

you need two sleeves (one with pattern right side up, and one with pattern face down) same with the collar.

I was using a brushed cotton twill fabric, and decided I liked the unbrushed side for the outside face of the jacket. So I didn't get confused I marked the wrong side of the fabric. I cut the sleeves, front and back pieces long and planned to adjust the length later when sewing.

also be sure to mark all alignment notches lightly with the sharpie on the wrong side. Most of the marks are within the seam allowance, so they should not show on the finished garment, but still be careful, if you're too heavy these marks may bleed through the fabric. Also mark where the waist line falls on the seam allowance of the back, and front pieces .

Because I was sewing in buttonholes, I extended the top of the right shoulder and added a flap along the right side of the outer front facing. These extension would be folded under and make the buttonhole fall on areas with a double layer. I marked the original patten lines So I would know where to fold.

Step 6: Construction

I stopped taking pictures for most of the construction but it is pretty straight forward.

line up the notch markings on the shoulders and the waist lines and pin together the back, the inner front facing, and the front facing (inside out). A 5/8 inch seam allowance was used throughout the pattern.

For the structural seems I used heavy duty thread. For finishing the edges of the fabric I used all purpose thread and a zig zag stitch as seen in this video

Sew the shoulder seems first
Sew the beginning of the side seams - from the armpit down a few inches

You should be now be able to flip it right side out and wear it like a poncho.

Find hem length (just bellow the knees) and where the side slits should end (I decided to end just about my where my pants sit), mark with pins. The hem should curve up gently and meet the sides at a 90 degree angle. Pin in place. Iron flat when you're happy with it.

trim bottom hem with 1+1/4 inch extra. Zigzag/serge bottom and up sides up to where side slits stop. Top stitch bottom with 1 +1/8 inch margin. Top stitch side slits with 1/2 inch margin.
turn coat inside out again
finish sides seams down to side slits

Step 7: Construction Continued

Sew the first inch or two of the sleeve seems. I did this because I thought it would make pinning the Sleeve to the jacket body easier.
Pin a sleeve in place by lining up the notch markings. Use lots of pins to distribute the excess fabric and to minimize any puckering. Sew sleeves in place.

Find sleeve hems and mark with pins. Leave 1+1/4 inch excess, zigzag/serge end of sleeve. fold into sleeve and iron flat. Top stitch with 1+1/8". I needed to do this before finishing the rest of the sleeve seam because I don't have a free arm sewing machine, so the finished cuff would not fit around my sewing machine.

finish sewing sleeve seems.
Zigzag/serge any remaining edges with AP thread.

On the right edge of the front facing fold 5/8" under and iron flat. If you are going to use interfacing to reinforce the buttonholes now would be the time to do it. The interfacing should be between the two layers of fabric (if you chose to add the flap and extend the shoulder). Finish with top stitching 1/2" inside seam.

Step 8: Collar and Back Tie

Collar
Perhaps due to an error in creating my pattern piece, my collar came out a bit long. I pinned a piece of the color to the jacket to see if I needed to trim it to size in terms of its side to side length. If you do this remember you will loose 5/8 x 2 in the seems when you sew it, and there should be some overlap where it will button together.

A change I made to queza7's instructions, before sewing the two collar pieces together, Zigzag/serge the bottom edge of one piece. Fold the bottom edge under 1/2 inches and iron it flat. Top stitch just inside the folded edge. Now sew the two collar pieces together with right sides facing in (5/8 seam allowance), sew just the side and top seams. Trim the corners and zigzag/serge the edges, Flip collar right side out and iron flat.

Since I trimmed an end off my collar I did not use the alignment marks attaching when attaching the collar. Instead I started from the outer front flap edge and pinned the inner layer in place. The fabric form the jacket body should be sandwiched between the collar layers. Continuing pinning the collar around the front and back. Check to see that outer collar will cover the collar/jack body seam. I used a zigzag stitch to sew the inner collar to the jacket.

Back Tie
The Pattern - A direct quote from the queza7's instructions:
//
1. Measure your waist. Add 6 to this measurement (we'll call it coat waist size).
2. Measure the waist of the coat pattern front and add it to the waist of the coat pattern back.
3. Subtract the coat waist size from the total pattern waist size (we'll call this cinch).
4. Subtract cinch from back pattern waist measurement and add 1+1/4 (we'll call it tie width).
5. The pattern piece will be 3+1/4 tall and the tie width wide.
//
With right sides facing in sew the two pieces together (5/8 seam allowance) . The original instructions
say to sew a U shape, the top, one side and bottom seam. Trim corners and turn right side out using the unsewn side seam. You can then iron it flat and slip stitch the remaining side seam. I never got the last side seam to look very good so I recommend sewing all the seams, leaving a small gap a few inches wide along the bottom edge to turn the piece right side out. Iron flat and slip stitch the gap shut. I feel that will be more hidden than the using the unsewn side seam.

Step 9: Buttonholes

Highly Recommended Tool:
Singer Automatic Buttonholer - They don't make these anymore but they are great. The old models still work on newer machines, even on some other brands. You can make keyhole buttonholes, and it is a 1 step sewing process. Here are two good pages about them One and Two

I made buttonholes 4 across the shoulder and collar, 3 down the side and 2 on the tie across the back. (I guess the real costume is 4, 4, 2)

make sure to make a test piece with some scrap fabric and interfacing if you used it. It is a good idea to get some practice using the buttonholer alignment guides and to see the effects of the stitch width selection. I had to fine tune my tension and adjust the stitch width on my buttonholer. Also test passing a button through the hole to make sure you have the right size buttonhole.

Chalk a line to make the aligned edge of the buttonholes. Mark where the first and last buttons will go. Evenly divide the spacing with the remaining buttonholes. I made 3 continuous, overlapping passes on each buttonhole which gave them a defined outline. Cut the thread long, so you can thread it through a needle and pull it through to the wrong side, then you can tie it off and trim it close.
Use a sharp utility blade or razor to cut the buttonholes open. Go slowly since if you cut too far you will ruin you jacket.

attach buttons, I found this video helpful.

Step 10: Pocket

I used this this Caduceus shape - by Retoucher07030 on deviant art. I removed some of the smaller feather lines above the snake head since I didn't think I could capture the detail in thread. My print size was 2 inches tall.

You can see my embroidery from digital artwork instructable here

remove from hoop and iron flat. With chalk, mark a box 5 inches wide and 4 inches tall. Draw a line through the center of the box, extending 1+5/8 inches bellow the box. Connect the end of this line to the two bottom corners of the box. Cut out pocket. You may wish to extend the top of the pocket so you can play with the size of the finished pocket and where the logo will fall on it.

Zigzag stitch/serge all edges with all purpose thread. fold the top edge under, press the fold with an iron and top stick across the top of the pocket. fold side and bottom edges behind pocket and iron edges flat. Pin to jacket and top stitch in place.