If you collect stamps and want to take the envelope backing off of the stamps you save from the mail, this is the way to do it without ripping the stamp and ruining it completely. Then you can press and save the stamps in your stamp collecting album without messy envelope edges sticking out. You'll also avoid having to cut off part of the stamp to make it fit in the album.
Cut the stamp from the envelope, leaving some of the paper envelope all the way around the stamp so you don’t cut any of the stamps edges off. The envelope backing will be removed with this process, so don’t worry about perfect straight cuts. Just don't cut the stamp.
Fill a large mixing bowl with warm water. Don’t make it hot or you could damage the stamp by removing too much of the backing.
Place the stamps with the bit of envelope backing in the bowl of water with the stamp facing down, so you only see the paper envelope backing.
Leave the stamps to soak and absorb the water. Be patient, as each stamp may take a different amount of time to loosen from the envelope. As the glue dissolves, most of stamps will come off by themselves and sink to the bottom of the bowl. Thicker envelope backings will take a longer time to absorb the water to get to the sticky glue. If the backing doesn't come off by itself, carefully pull the wet stamp from the wet backing. Be aware that stamps are much more fragile at this point, so remove them carefully from the bottom of the bowl.
Spread a dish towel open on the counter. One by one, put the stamps, picture side down, on the dish towel and let them dry completely. If you put stamps sticky side down, they will catch and stick to the threads of the dish towel as they dry. Stamps will curl up a little bit as they dry. Once the stamps are totally dry, place them in a stamp album where they will be pressed flat again.