The first question we must ask ourselves is whether we really want to remove the stamp from the paper it’s glued onto. In most cases, if a stamp is on a full envelope that is old, it probably has more value staying on the envelope. However many people have collected stamps over the years by tearing off the corner of the envelope with the stamp on. So this is what we are dealing with here.
Take a shallow dish and fill about 3/4 with cold tap water. Place the stamp face up on the surface of the water so that the surface of the stamp has a chance to stay dry. By doing this, you reduce the risk of colours running (which could happen with older stamps), or the face of the stamp becoming wrinkled. To do it properly, you should only soak one or two stamps at a time.
If you leave a fairly large border of paper around the stamp, you can easily see when the backing paper is really soaked. At this point carefully lift the stamp out of the water (plastic tweezers work well here) and hold it face down on a flat surface. Peel the paper off the stamp, not the stamp off the paper. If you can’t, carefully put the stamp back into the water for a few more minutes.
Place the stamp face down on a piece of white blotting paper or kitchen towel at room temperature. When dry, place in a thick book and leave for a few days.
Although I have mentioned this method, it is not recommended. It cannot be done for large quantities, it steams up the house, the stamps can become curled, there is a risk of burning yourself, and there is also a risk of damage to the front of the stamp.
Be very careful with very old stamps. Many were printed with fugitive ink, so that the colours would run if anyone tried to remove the postmark and re-use the stamp.