One pair of jeans that fit you well before you were pregnant
One t-shirt that fit you very snugly before you were pregnant (maybe even a size too small)
Scissors, seam ripper, pins
Sewing machine, coordinating-colored thread
Piece of soft elastic long enough to go around your hips or above your belly
Step 1: Start with jeans and a t-shirt.
Step 2: Cut the top off of the t-shirt just below the armholes.
Step 3: Unsew the bottom of the back belt loops using the seam ripper.
Step 4: Cut a curve across the front of the jeans.
Be conservative here. You can always cut more off if you need to. Try to cut across areas that have less fabric/decoration (e.g. above the coin pocket, but below the waistband/belt loops).
Pin the fly closed and the pockets in place.