Now that you have a shoebox full of collected stamps, soaking off is next on the list.
Care should be taken here for two reasons. Firstly, ensure that the stamp does not lose value by being removed from the envelope. To some postal history collectors, an old, clean, addressed envelope is worth more than the value of the stamp it bears, especially if the handwriting is particularly attractive. Or, if the stamp has a particularly interesting postmark, it's worth keeping the entire piece. Stamps should also not be removed from First-day covers.
The second reason for taking care when soaking off, is obviously not to damage the stamp. Stamps should never just be torn off their backing paper. When I first started collecting I used to place the stamps face upwards on the surface of a bowl of water until they could be easily peeled from the paper. They were then laid face downwards on blotting paper. This is the safest method to use. Older, frail stamps, and those which have inks which can "run" when exposed to water, should just be placed face upwards on damp blotting paper and left for several hours before any attempt is made to peel them.
You will inevitably find that once the stamps have dried, they will have curled. This is because the remaining glue on the back has dried and shrunk to a different extent than that of the stamp itself. Carefully pressing the stamps flat again by means of a heavy book is one solution.
With modern stamp adhesives I no longer bother placing the stamps carefully face upwards. They are simply immersed in a bowl of luke-warm water and left for a while. Once they start to separate from the envelopes, I carefully remove them and place them face up in a special "Desert Magic" stamp drying book. Once the book is full, I put a couple of heavy books on top to ensure the stamps don't curl, and leave them to dry.
I've found the best way to reduce curling is to ensure that all the adhesive is removed from the back of the stamp before it is allowed to dry. Once the stamp has separated from its backing paper, I keep the stamp immersed in water whilst gently rubbing the back of the stamp with a finger to remove the glue. Obviously I don't do this with older, more fragile stamps.
It's also advisable to sort the stamps into separate piles before the soaking process, and treat each pile separately. This prevents the entire lot from becoming stained due to dye leaching from coloured envelopes or water-soluble postmarks.
The first pile should contain self-adhesive(*) stamps (the glue on these takes much longer to soften).
The second lot should be those on brown or coloured envelopes.
Then the stamps with red, purple or other dark coloured cancellations.
Finally, everything else.
In order to cut down on the amount of soaking required, it's best to cut the stamps from the envelopes first using a pair of scissors. Leave a fairly wide margin around the stamps to ensure the perforation is not damaged.
Some stamps will not soak off with water (like the first USPS self-adhesive (*)issues). A specialist liquid called can be used for these - or instead just leave the stamps on the neatly-trimmed backing paper.
* Some sources recommend that when it comes to self-adhesive stamps, these are best left on the envelope. Mint self-adhesive stamps must be on their original backing paper.