Old pair of jeans (or just the top waistband with zipper part)
Plain crew neck sweatshirt
Step 1: Cut the Jean Waist and the Sweatshirt Collar
Cut around the waistband and down around the zipper, making sure to leave enough room to sew. If the jeans have a logo on the back, rip that off with a seam ripper.
Take out your sweatshirt and carefully cut the collar off right below the seam.
Step 2: Pin the Jean Waist to the Sweatshirt Collar
Cut right next to the back belt loop.
Flip the shirt over. Pin one of the jean pieces to the collar.
Take the other jean piece and lay it on top, making sure you line it up. Pin it down through all the layers.
Cut the excess of the piece off, a little bit beyond the center back of the shirt. Do the same for the jean piece underneath so that it doesn't get too bulky back there.
Step 3: Sew Jean and Sweatshirt Together
Now, sew around the bottom of the waistband. Then sew down the bottom part of the belt loops.
Turn the sweatshirt inside-out. Cut next to your seams that you made around the zipper fly.
Turn it right-side-out and yer done!