Most bathing suit tops come with foam inserts to help shape and cover the breast. If the original padding gets lost, the top is rendered almost useless. Luckily, inserts are sold separately online and in fabric stores, and can easily be placed and sewn in to keep them secure.
Lay bathing suit top face down. Locate the small slit where the padding should be inserted.
Orient the insert right side up with the convex side facing up and roll tightly from the top down to create a cylinder.
Insert the rolled up padding into the slit. When completely inside the bathing suit, rotate the cylinder so the bottom of the insert lies at the bottom of the cup space. Carefully unroll the foam. Repeat with other insert, removing any old padding, to create matching cup shapes.
Sew the new inserts into the bathing suit to ensure that the padding will not move around or need to be replaced again. Simply tacking the insert at three points (top and bottom sides to form a triangle) will hold the padding in place and will not be highly visible on the outside of the suit.