Costumes take on all shapes and sizes, literally. The easiest way to create an effective female fat suit is to start with the appropriate foundation garments. You can shortcut much of the sewing by combining two leotard-style body suits. The fat suit requires very little sewing and can be done in a short amount of time.
Select two leotard-style body suits. The inside suit should fit your performer. Have her try it on and make sure that the size is comfortable. The second suit should be at least four or five sizes larger. Its best to choose garments that are as similar as possible.
Place the smaller body suit inside the larger one. Machine stitch, using thread as close to the suit colour as possible, along the bottom edges of the buttocks area. Measure to the centre of the buttocks and mark a line up from the crotch to the seam above the buttocks. Stitch the two suits together along the line to form two hip-shaped pockets with the bottom, sides and centre stitched and open at the top. Fill the two hip pockets with cotton batting until they round out to the shape you want. Stitch the hips shut along the top.
Stitch along the bottom of the bosom area following the shape of the cup. Stitch the top of the cups together, leaving the centre 2 inches open. Fill the bosoms with cotton batting until you have the size breasts you want for your costume and stitch the top of the cups together.
Sew the suits together along the bottom of the crotch, up the front of each leg to the side seams and up the side seams to 4 inches below the breasts. Fill the belly with cotton batting and stitch a curved line across the top of the belly arching downward to give a round shape to the stomach.