If you are using the cold process method, you will want to let your soap dry and harden a bit before you stamp it. If you stamp too soon, the soap still be sticky and soft and the resulting imprint won't be clean and neat. If you wait too long, the soap will too hard and it will crack. Most people wait between 2-5 days after cutting before stamping their soap bars, and some get great results after a week or two.
To use the stamp, press it down firmly, and if it is a large image, press on different parts of the stamp to make sure the whole image was pressed in. Then very gently and slowly, pull the stamp out. Pulling quickly could tear the soap.