So this is all the steps written out! I'm pretty sure if you click it, the image will get larger. If that doesn't work, try the link on the right. The rest of the steps will be pictures of my version, which is a little more advanced.
I cut two inches off the bottom of my shorts and used it as a casing. Three layers- shorts and sweater sleeves, good sides together. Good side of casing to the wrong side of the sleeve.
Flip the casing up to cover the raw edge of the sweater and bottom of the shorts.
The finished product! I tried to make the shorts section short so I could wear them with just a long shirt.