This is SERIOUSLY cool.
Step 1: What You Will Need
Scissors/Stanley Knife (or Xacto knife, I think, in the US).
Needle and thread (preferable similar colour to the stitching of your jeans, but it doesn't matter. White, yellow or blue look the best). Or a sewing machine, if you know how to use one.
Something to mark your jeans with (optional).
An old pair of jeans that you no longer want to wear (or that you don't mind looking like a prat in afterwards).
A belt that you don't wear.
Step 2: Measure and Cut
It's better to cut from the leg of your jeans, as it is the longest part. And extra denim you need you can just cut from the other leg, as the seam and style will be the same.
Once you've seen how much of your jeans you are going to need to cut it out. To give the belt a little bit of strength and a little bit more to look at, I cut a belt's width either side of the seam, but this shouldn't be necessary. After cutting out as much as possible from one leg, I had to steal six or seven inches from the other leg.
The cutting doesn't have to be too neat, as it can be straightened out when sewing. If you do want it to be neat, mark the jeans with chalk or similar. You can use a pencil to create a crease and an easily washable line (if you use a very soft pencil).
Step 3: Start Sewing!
So after spending thirty years threading the needle, I began. Then I got bored and started going as quickly as I could because it was taking forever. This is why a sewing machine would be useful. You'd have to pin it all up first though, as it can get tricky if not neat (_).
Basically, find the side you'd like to be the front, ie the one with the seam, then fold it so that the seam is on the edge (or middle or wherever, mine sort of dips up and down a bit ). On the back, you just have to fold one edge towards the other but not all the way, then fold the remaining bit over and sew, making sure you're catching all three layers. Sorry for the lack of a picture for this step, I didn't want to jeopardise my progress. If you want to make the stitching as regular as the one already there, go for it. Just make sure you take breaks for food, I don't want to be responsible for your death.
The second picture is where I have joined the two different parts of denim, before i tidied it up. It's simple, all you need to do is cut the second one slightly bigger if you can, but it doesn't matter if you can't. Then simply fold it over and continue stitching. You can revisit this join when you've stitched the whole length, as it might need a little bit of added strength. Try to make it as smooth as possible, or you'll have trouble getting it through the belt loops in your trousers/jeans(/pants) and also back through the belt buckle.
Step 4: Remove and Attach Belt Buckle
If you don't want to dismantle your belt or don't have a belt buckle, you can use two metal rings instead and sew a loop of denim (you could use a couple of belt loops to make this extra cool!) through those on the end, or alternatively you can buy a new buckle. There are most likely other ways to do this, so take your pick. Cheap belts are usually easier to take apart, btw!
Also if your belt has a little metal clip on the end, take it off using your extraordinarily strong fingers/a set of pliers and reapply to the other end of your new belt. This will make it look better and also make it easier to use. You can see this on the finished product.
Step 5: Finished!
Thanks for reading my first and best Instructable! ;)
If you want a little more strength from your belt, you can place a strip of cardboard or similar before sewing up. You could even do more denim for a more authentic denim belt.
You can use a button system to close up your belt, but this doesn't look so good and can be quite impractical.
After a little use, frayed threads may appear. Just cut these off as close to the 'root' as possible and you should be fine. I'm not quite sure how to combat this, but if need be you can always create a new one!
When using pliers to close the metal clip or for any other thing you don't want to scratch, use a cloth or paper (or denim) around the clip so that the pliers don't mark it.