If one of the parts is too small, pinch its edge and pull it out gently. Work gradually. If you just want to make the edges sharper, just pinch it all around (don’t pull out). If the parts don’t fit together at all, consider remaking them.
Apply a small amount of glue and stick the two parts together. If you apply too much glue, the parts will slide around and the glue will ooze out.