Remove the old insole. Depending on how old the shoe is, this is usually done quite easily. Reach inside the heel side of the shoe and pull up at the end of the insole. It may stick, but it will come out with a little force. Once you get a corner of it, tug on it firmly and start to pull out the insole. Once half the heel part is pulled up, it will easily slip out. Set it aside.
Clean the inside of the shoe with a damp rag before putting in a new insole. Wipe off any residue left from the original insole or any dirt that may have gathered at the corners.
Make sure the new insole fits your feet and shoes correctly. Measure it up against the old insole by placing them together with the outside bottom sides touching. Most likely, the new insole will be slightly bigger (it's better to have it bigger than smaller). If it's too big, cut it to the same size as the original.
Start with the toe end of the insole and gently push it inside the shoe until it can't go any farther. Then push down the middle to heel part of the insole so that it fits the shoe exactly. If there is any part of the insole that is sticking up, take it out and try again. It is essential that the insole lies flat against the inside of the shoe.