Try on your shorts. The easiest way to find the length to cut your old jeans is to compare them to a pair of shorts whose length you want to mimic. These days there is no wrong or right length to wear, so try on the shorts you already have and pick out a pair, based on its length.
Lay your jeans down on a flat surface and fold them in half lengthwise so the legs are on top of each other. Do the same with your shorts.
Place your shorts on top of your jeans, making sure the crotch of your jeans and shorts match up with each other and are flush.
Take your scissors and cut off the top leg of your jeans, using the shorts as a guideline.
Take away the shorts and then use the leg you just cut as a guideline to snip off the leg folded beneath it.
Check out the length of your new cut-offs by trying them on. If they're too long, mark a line with chalk, to use as a guide for cutting off a bit more.
Run the blade of your scissors across the cut-off edge of your shorts. Do it in fast, quick motions, making sure to repeat in the same direction (rather than just rubbing it back and forth). This is going to help release some of the threads in the shorts.
Pull at the threads with your index and forefinger, to separate them from the rest of your jeans. Finally, give the edges a worn-in look by running an emery board back and forth across them.