Does your unmounted stamp have a sticky foam on the back of it already? If it does, just peel the paper backing off and apply the stamp to your clear block and stamp. If not, you can either buy a sheet of EZ-Mount foam and add it to the back of your stamps, or add double stick tape to the back of your rubber before placing it on your block. Check this thread
in our forum. It's full of more options to make unmounted stamps stick.