Some advice for cutting your shorts: 1) start out longer than you think you want the shorts to be 2) use a marker/chalk to mark your jeans where you want to cut them while you have them on. It is also helpful to use another pair of shorts as a template.
STEP 1: Pin the lace to the edge of your shorts. The edges of my cut offs were already folded up so I decided to leave a fold, but I think it would also look good without a fold (that was my original plan). You should experiment a little as you start pinning--does it look better folded/not folded? does it look better with the trim sewed on top/under the edge of the shorts? This all depends on the trim you're using. My trim was really thin and I wanted to be able to see the whole design, so I decided to sew it on the top side of the edge (the top of the trim also hid my seam nicely). Pin the trim down every couple of inches (make sure it's nice and straight!). When you get to the end leave a bit of extra trim on either end so that it can be folded under at the end (you can always cut off excess trim, but you can't add it back on). I think it looks best if you make it so that the ends meet at the inner thigh seam of the shorts.
STEP 2: Begin sewing at one end (I used a machine, but I'm sure this would be really easy to hand stitch). Go slowly to make sure your trim stays straight and doesn't bunch. When I got to the end I just folded the end of the trim under (and tucked the other end under it) before I sewed over it to hide the ends.
STEP 3: My ends looked a bit awkward at the end, so I decided to hand stitch the two ends together.
This project was so easy and I love the end results...I'm pretty sure I'm going to be doing a few more cut off makeovers this summer!