I recently cut off the legs of one pair of my jeans and turned them into shorts. For a while, I wasn't sure of what to do with the legs that I was left with. Then on day, I found myself with a lot of time and nothing to do with it. So I made a pencilcase.
And it's really very simple. Basic sewing skills are required, and it's probably easier with a sewing machine. Don't be disheartened if you don't have one though, because I dont't either and this was finished in an hour.
Good luck to anyone who tries this, and thanks in advance for clicking on this instructable (even if it was by accident ;D)
And I do apologise in advance for the dodgy camera work, I lack a camera. (SORRY! :( )

Step 1: What Dost Thou Need?

- Leg of old pair of jeans, or just any bit of fabric really
- Lining fabric (thin, plain weave cotton will do.)
- Basic sewing equip, i.e. needle, thread, scissors, measuring tape, pins
- 20cm zipper
- Decorative stuff...?

For me, all this stuff was already available at home, so this project didn't cost me a thing. However, you may hve to purchase a zip, which shouldn't cost more than $3 really.

Step 2: Lining First

Most pencilcases are 20cm long, and so measure the width of leg before you do anything to it. Luckily for me, my leg was about 21cm wide, so it was a perfect fit. If your leg is too thin, you may want to undo the side seams, at a strip of fabric in betweena nd resew it.
I cut a strip of leg (?.O) about 15cm long. Your's could be longer or shorter, whatever is to your taste. But remember to take into consideration seam allowance and whether or not you want a gusset.
Then I cut a strip of lining, 15cmx45cm (the extra 5cm was to allow for the seams).
Pin the short ends of the lining together, and sew. If sewing by hand, I suggest using backstitch, because it's stornger. Trim off excess fabric. Then sew along one longer side, again using backstitch. Trim off excess.
Now, if you want a gusset, (where the bottom is flat, making the bag/pencilcase wider), take one corner and line up the side seam with the bottom seam. (Refer to images if confused). This should create a triangle. Fold out the seams, pin 2cm from the tip of the triangle. Then sew straight across, where the pin is. Repeat on the other side, but line up the fold in the fabric, rather than the seam.

Step 3: Denim Next

As I didn't undo the side seams from the leg, I just sewed along the bottom. The problem that i did have was that one side of the fabric was shorter than the other, meaning that the seams were closer together on one side, and that the fabric didn't fold out evenly. To compensate for this, simply choose one seam to remain the same, and then find the other middle and fold, like in the second image.
Repeat the corner thing with the same measurements.

Step 4: How to Sew a Bow

If you wish to add any decorations, now's the time to do so. I decided to make a simple bow to attach to the front of my pencil case.
So you take a rectangle of fabric, as wide as you want the bow to be and twice as long. Fold it in half to make the bow size that you want, and sew almost all the way around, leaving a gap big enough for you to turn the rectangle inside out. Trim off excess fabric before flipping, and trim the corners too. Then sew up the gap using a slip stitch (here's a how-to).
Find the middle of the longer side and pinch it together. Sew a few small stitches on top of one another to keep the middle pinched together, giving the rectangle a bow look.
Now take a strip of the same fabric and fold it into three lengthwise, hiding the edges. Wrap it around the pinched center once tightly, and sew it together (image 7). Cut off excess fabric, and smile at your handiwork.

Attach the bow to the denim, being careful to keep the side where the stitching shows next to the denim, with the no stitching side showing.

Step 5: Lining + Denim

Look closely at the way I've put together the lining and the denim. See how both the right sides are facing out? See that? Well that's wrong. (Woops!) I made a really noobie mistake, and so please do as I say, and not as I do.
KEEP THE RIGHT SIDES TOGETHER.
Line up the side seams-this is most important in keeping the gussets together. Then go ahead and pin up the top. Sew all along the top, but leave a gap to turn everything through. Don't forget to trim off excess fabric!

Step 6: Attach Ze Zipper

I know that it's possible to sew the zipper, the lining and the outer fabric altogether at once, as to show no stitching, but to be honest, I wasn't bothered enough to spend 10mins lining up everything and figuring out where everything was supposed to go. So I attached the zipper here, at this stage.
As the zipper was only 20cm long, and the top of the pencil case was 21cm long, there was some gathering required. But that may not eb necessary for you. First, slip one end of the zipper into the gap that was intentionally left for turning the pencilcase right way 'round. Then pin the pencilcase to the zipper, with the zipper unzipped. Try to gather the fabric evenly, if necessary. When it's all pinned up, try zipping and unzipping a few times, just to make sure all is fine and dandy. Then go and sew. :)
If you're using a machine, use a zipper foot. It makes life soo much easier. if you're sewing by hand, remember to keep the thread taut, and don't leave any stray loops of thread (like I accidentally did). Loops of thread get caught on the zip, which is really irritating. So just take this bit slow, it'll be worth the effort.

Step 7: Done! Afterthoughts...?

After the zipper is attached, you're finished! So why is there another step? Well, I kinda just wanted to talk about it.
Remember to always be careful when you sew. Check, check again and triple check. Don't be like me, and just check 1 and a half times. Not good enough XD.
Sew, um, I hope you've enjoyed reading this instructable, and I also hope to see some other versions popping up somewhere...:)
Don't be afraid to comment, question or suggest. Please do :)