You need to make an environment of humidity around the envelope to release the glue. However, you also have to take care that there is no direct contact of the envelope with the moisture, else it will be ruined. A plastic box will keep out the air and will be able to create the environment you need. The box that you choose should have a tight fitting lid. Place the box in a location where it would not be disturbed for a day.
Soak up the sponges in clean water. Now place them at the bottom of the box. Over the sponge place the metal gauze. The gauze acts as the divider between the stamps and the sponge so that the water does not damage them. Place the stamp/envelope over the gauze. Place the lid over the box tightly.
Check on the sweat box regularly. Make sure that humidity is building up. If it does not, you might have to re-wet the sponges to increase it. The stamps should come loose after a time of 24 hours. Give them some more time if they have not. You will have new additions to your collection soon.