Once braces come off, teeth automatically try to shift back to their original position. A retainer keeps them in place and must be used as prescribed; otherwise, the teeth will shift and the retainer won't fit. If there's pinching or pain, or the retainer won't stay in, make an appointment with your orthodontist.
Determine which kind of retainer you're using. "Hawley" retainers consist of plastic that fits snugly against the inside of the teeth, with a metal bar over the outside, keeping the retainer in place and the teeth from shifting. Invisible retainers, called "Vacuform," cover all the teeth, front and back, using suction to stay in place.
Move the plastic section into place against the backs of your teeth and set the metal bar over the front if you're using a Hawley-type retainer. There should be no space between the teeth and the plastic; a little looseness or gap isn't a problem as long as the retainer doesn't shift around. If the corner of the plastic is poking into your gums or causing irritation, smooth the edge with a nail file.
Place the retainer over your teeth if you're using a Vacuform retainer. It should fit snugly, and the pressure should be fairly uniform. If there's pain, or the retainer won't stay on by itself, make an appointment with your orthodontist.
Make sure that there's no food or debris in the plastic part of your retainer. If the inside is dirty or something is trapped in the plastic, scrub it with a toothbrush or soak it in a retainer solution. Don't boil your retainer because the plastic might melt.