Use a piece of soft or spongy foam for best results. Purchase some from a craft store, or remove some from a bed or throw pillow. Use scissors to shape the foam to the size you want. It's best to start big because you can always trim it down later.
Sew some soft or stretchy cloth over the foam. Sew a piece of elastic to each side of the fake pregnant belly to hold the belly snugly to your front. Measure the elastic before you sew to make sure the belly will fit snugly. To make the belly fit more closely, sew on two pieces of elastic--one across the top and one across the bottom of the foam belly.
Wrap your piece of foam to your body with gauze bandage or plastic wrap if you don't want to sew cloth over your piece of foam. The bandage or plastic wrap will smooth out any uneven spots in the foam and mold the shape to your body. Wear a soft, close-fitting shirt over it and a looser-fitting shirt that shows the roundness of your belly.
Try a partially inflated beach ball if you can't find a piece of foam. It won't be as heavy as foam but it can create the same rounded effect. Wrap it to your body using material, gauze bandage or plastic wrap. Wear a soft, close-fitting shirt over the belly and a looser shirt over that to display the belly well.
Practice walking like you're pregnant. Foam fake pregnant bellies are usually best for creating some weight. It's easier to walk or waddle like you're pregnant if you feel pregnant.