This might not be the best DIY you've ever seen. I'm not very exact when it comes to...well, basically anything that isn't a list. I'll try to walk you through what I did, though, and best of luck to you in your endeavors! When I was trying to figure out how to start, I briefly looked at these three sites for help:
First, find a pair of jeans to cut up. Try them on, figure out how long you want them to be, and mark that spot with chalk.
To do the scalloped edge, find a circular object you can use to trace a makeshift pattern on your jeans. You could also cut out a paper pattern. I definitely recommend doing it Mandi's way.
Usually when I chop my jeans into shorts they end up slightly crooked, because I'm way too lazy to measure anything. However, I wanted these to be cute, so I measured the inseam and outseam on both sides to try to get them to match up. (Note: I'm terrible with this kind of detail, so naturally I messed up and had to adjust my shorts later).
Voila! Try them on to make sure they look okay, and adjust as necessary.
If you want to add an extra touch of girly, find some lace - I found mine on a thrifted pillow case - and pin it around the inside edge.
Carefully sew the lace onto your jeans. Again, I'm not really meticulous when it comes to craft projects, so if you look closely you'll see that mine are a little sloppy.
Take silly pictures of yourself wearing your cute new shorts!