Step One: Sewing The Shorts
Place the two front pieces of the shorts right side together and sew the crotch line at 3/8″ seam allowance.
Take the back pieces of the shorts right side together and sew at the crotch line.
Sew the sides together and zigzag or use your serger. Sew the inseam and zigzag or use the serger.
Step Two: Sewing The Top
Sew the darts on the front of the top and iron them downward.
Sew the sides of the top and zigzag or use your serger. Pin the top to the shorts (right sides together). Sew at 5/8″, zigzag or serge the seam allowance, but do not cut. You need the seam allowance whole because this is where you will be sewing the elastic. Cut a piece of elastic the size required for your waist. Do this by wrapping the elastic around your waist to a comfortable tightness and adding 5/8″ to sew. Fold the elastic in half and mark the two folds. Fold each half so you end up with four well-distributed marks on the elastic. Pin this elastic loop on the sides of the shorts and on the front and back seams.
Start sewing at the back and pull between pins. This technique will gather the romper around the waist evenly and prevent you from getting irritated by the elastic.