Summarize the following:
You don't really need much water, but try to put an inch-and-a-half in the bottom of the pot, and put it on the heat. If you use too much water, it will take forever to boil, but don't use too little, or it'll evaporate before you can open the envelope. As you wait for the water to boil get ready with your envelope. With the side of the envelope with the opening flap facing down towards the water, find a spot where you can gently ease your thumb under the flap. A good spot is at the very end of either side of the flap, because some envelopes don't have any glue there. Be gentle, you don't want to tear the paper. Once the steam saturates the envelope (it will feel hot, damp and limp), the glue will melt, and the envelope will open.  Don't hold the paper in the steam for too long. You'll get it all soggy and give the game away. Hold the envelope over the steam for about 15 seconds, then start trying to open it, returning to the steam if it is still stuck.  Consider using a skewer instead of your thumb or finger to peel the flap up. It allows for more precise control and is especially effective if you place the skewer vertically underneath the flap at one end and twirl it along the length of the flap.

Summary:
Put a pot of water on to boil. Hold the envelope over the boiling water. Apply constant light pressure on the flap.