Most of the time, the patches that come in a pre-made kit will have a thin foil backing which you will need to remove to expose the adhesive. Take that backing off, and apply the patch directly over the hole, pressing it firmly down to seal it onto the rubber cement.
If you do not have a patch, you can try using a piece of another old inner tube cut to the right size if you are really desperate. You will need to use sandpaper to roughen it, since it will not have the same adhesive as the patch from a store-bought kit. It is more difficult to get these patches made from inner tubes to stick and hold, but it will usually be enough to get you home.