This is a very easy project if you have a sewing machine and basic sewing skills.
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.
The jeans should have fit you well before you were pregnant, and the t-shirt should be very snug.
Step 2: Cut the top off of the t-shirt just below the armholes.
Discard the top (sleeves and neck) or save it for some other project. Keep the nice tube bottom.
Step 3: Unsew the bottom of the back belt loops using the seam ripper.
Make any adjustments you want to the jeans. I never liked these pocket flaps, so I am taking them off (with the seam ripper as well).
Step 4: Cut a curve across the front of the jeans.
First, pull the zipper closed. Then, cut across the front of the jeans: start about 1/2 inch below the waistband on the side, curve down to about half-way down the length of the zipper, then curve back up to 1/2 inch below the waist band.
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.
Step 5: Cut straight across the back of the jeans.
Cut about 1/2 inch below the waistband. This is why you unattached the bottom of the belt loops in step 3.
Step 6: Machine baste across the front curve.
This is to hold the pockets, fly, and zipper in place. Do this part carefully so that you don't break a needle on the zipper.
Step 7: Put the jeans on to check the fit.
They should sit just below your pregnant belly (or where you think it will be).