To start out, you will need yarn, scissors, and a pencil or other thin dowel.
Step 2: Setup
Cut the yarn until you have 24 strands. Then wrap them around the pencil or dowel. Use a slipknot or something similarly easy to undo for the yarn above the dowel, then tie or clip it to something to keep it from moving around too much.
Step 3: Starting out
Bring the first strand of yarn under and over the other strands. You can do this quickly by separating every other strand, then pulling the first one through. I am going from left to right, but going right to left will give you the same look and might be easier if you are left handed. Once you pull the next strand through, the previous strand goes over it.
Eventually you will see the diagonal stripes. Once you are done with one half and are ready to remove the pencil/dowel, it should be turned around rather than flipped over. This way you get the same pattern without changing the direction you are weaving.
Step 5: Finishing off
At the end of the bookmark or belt, you can tie it off several ways. For this one, I twisted the pairs together, tied a knot at the end, and trimmed off the ends to make it more or less even. You can also braid them, simply tie a knot, or whatever you like.