Put the smaller t-shirt inside the larger t-shirt. Match up the necks, bottoms and sleeves.
Stitch the sleeves and bottoms of the two t-shirts together around the cuffs and hems so that you have now made a great big t-shirt pocket.
Put the joined t-shirts onto a dressmaker's dummy.
Stuff batting in between the two t-shirts, through the necks of the fat suit. Fill out the front more than the back. Shape the body as you want the wearer to look.
Hand stitch definition lines, such as chest, belly, hips and buttocks into the fat suit.
Try the fat suit on the wearer.
Pin the t-shirts together at the wearer's crotch, onesie-style, to create legs in the fat suit. If possible, make a slight U shape at the crotch to give definition to the leg holes. Mark the crotch of the fat suit.
Take the fat suit off of the wearer.
Cut out the crotch area of the fat suit according to your markings.
Stitch snap tape to each side of the crotch. One side should be male snaps, the other female snaps.
Stitch the necks together to seal in the batting.