Onesies for infants act like shirts that never come untucked. They conveniently unbutton or unsnap for diaper changing. Although onesies can be found anywhere baby clothes are sold, they can be cheaper to make at home using a toddler T-shirt and a few common sewing supplies. A homemade onesie can give the baby a custom fit and can supply a way to repurpose old or unused T-shirts.
Choose a toddler T-shirt that is just big enough to reach from the baby's shoulders to her knees. Spread the T-shirt out flat on a table.
Measure the distance between the bottom of the baby's neck to the belly button. Use this measurement on the T-shirt, starting at the collar, to determine where the belly button will hit on the shirt. Mark this spot with chalk.
Lay an infant diaper over the bottom of the T-shirt, lining up the top of the diaper with the chalk mark.
Cut around the diaper to shape the bottom of the onesie, keeping 1 inch of fabric around the outline for seam allowance.
Turn the T-shirt inside out. Fold the cut edge under 1/8 inch all the way around. Fold it 1/8 inch more, and iron the folds in place. Sew the hem in place.
Turn the T-shirt/onesie right side out, and hand sew three snaps to the bottom of each side of the crotch area. Space them about 1/2 inch apart.