This was the 3rd Leather working project that I did and I use it to carry around my tablet along with other bits and pieces that I have. The inside pocket can really be customized to whatever you want!

Here is my first Leather Working Project to help me decide how deep into Leather working I wanted to go. https://www.instructables.com/id/Leather-Wrist-Cuff...
Here is my Second project, I knew I wanted to do a bag but felt I needed to practice some straight lines. https://www.instructables.com/id/Leather-Notebook-C...

Step 1: Make Your Pattern and Cut Leather

Before you start gather your tools:

Leather Fork
Hammer
Scribe to scratch lines into your leather
X-acto Knife
Double sided tape
x2 Leather Needles
Waxed Thread
Cutting surface
wood block or stump for puching holes with the fork and hammer
Leather punch for the Strap

I like to use a roll of brown paper from an art store to measure and cut out my pattern before scoring my leather for cutting. Use the image with measurements to create your Patterns.

Arrange your pattern on your leather, I had to buy half of a hide. Do NOT forget that you should use the longest edge of the hide for your straps if your doing long straps!

Be sure to gather all of your bags hardware ahead of time in case you need to customize your pattern. Not all buckles are the same.

Step 2: Optional Inside Pocket

Optional:

The inside Pocket can be whatever you want it to be. For my Messenger Bag I made these two 6.5 inches X 7 inch pockets for chargers and cables that I stitched to the main pocket "Ea". Use the "Ea" piece for the internal pocket and cut extra squares to add to the bag now if that what you want but as this pocket will be on the inside you need to finish it first.

The smaller pockets are 1 inch deep so I cut a 1.25 inch square off of the two bottom corners. 0.25 inches is for your stitching. Punch holes on the inner two sides of the notched corners so you can stitch the bottom of the pockets together.

Be sure to use double sided tape before punching the holes of the small pockets to keep them in place.

This inside pocket will stitch only up the sides, being sure to align the bottom edge with the bottom of Piece "A".

Step 3: Add Strap Loops Hardware

I purchased 1.75" Brass flat one side strap loops.

Take your x2, 5" strap pieces fold them in half with the flat part of the brass strap loop in the middle and use your double sided tape to line everything up. You should have three layers of Leather.

Now use your Fork to stamp your holes for threading.

Tip: When using a fork score/scratch a line in the leather to help line up the forks then use the last hole made by the fork as a pivot for the next row. This will help make sure your line is nice and straight.

Step 4: Putting It All Togeather

Now that all of your pieces are cut and your inner pocket and pieces "Da" and Db" are completed with the strap loops we can start putting the whole bag together.

Use double sided tape that is 1/4 inch wide to first adhere the pieces together. We will work on and stitch the full length of the bag first before adding the sides. your seams will all overlap by 1/4 and your stitch lines will be 1/8 of an inch in from an outside edge.

The "A" piece where the end was not rounded will connect to "B" along the 13.25 inch side with the edge of "B" being on top/ the outside. You will notice if you are doing the inside pocket that you will be stitching up the bottom of the inner pocket at this step. the outside edge of "B" should be visible on the outside of the bag.
The other end of "B" will connect to "C" along the 13.25 inch end again. to be consistent make sure that the edge of "B" is on the outside edge. Now the difference with the front of the bag is that you will also add "Eb" to the front so that it overlaps and is sandwiched between "B" and "C" edges with the same 1/4 inch seem.
Once the three have been stitched together you can punch your holes and stitch the pocket "Eb" up the sides. This is of course the from pocket.
Now that the length of the bag is all stitched you will want to check the sides to see where about they will line up and then measure down 2.5 to 3 inches for the placement of the inner pocket. You will, Score, punch and stitch 3 sides of the pocket.
I stitched the inside edge of the bag to the sides first, this way as I followed through with my taping and punching I knew if my measurements were off at all it wouldn't matter as much as my flap would simply be longer or shorter than I expected.
When I got to the bottom of the side pieces you will notice there was a 1/4 inch notch cut out to compensate for the transition around the curve. Keep your stitching tight here there should not be a big gap, just do the bottom edge of the sides then finish the back.
Once the bottom is attached tackle the last side pieces up the back of the bag. When you finish your stitch, back stitch 1/2 inch and tie two knots to finish it all off.