Hi,
cold water is fine for most stamps with gum (the ones to be licked). With self-adhesives it depends of the country of origin: Netherlands and Germany come off in cold water, UK, USA, France and others are somewhere between more difficult and impossible. For these I try boiling water, for some that works, if not I cut off the paper as close to the stamp as possible. The ink used for cancellation does (generally) not dissolve in water, so leaving the stamps for quite some time is not a problem.
Then there is paper and paper: some coloured enveloppe paper (typically used for Christmas greetings) will release the dye and the stamp becomes coloured. The stamp has to be in water as short as possible, so I use boiling water with a bit of kitchen salt, grab the stamp-on-paper with a tweezer, one at a time, completely immerge it in the water and move it until the stamp comes off, then rinse with clean water.
Drying and pressing: there are drying books like the one from Lindner or Kobra with blotting paper (vloeipapier) and interleaves of transparent foil:

These you should get from your local stamp dealer.
Take the stamp out of the water, put it between two sheets of paper towel to remove most of the water and leave it a few minutes, put it in the drying book with the gum side against the transparent foil, close the book and put a weight in order to press (5-7 kg of books or similar), leave the whole over night. Should against all odds the stamp still adhere to the foil, wait until it's completely dry and carefully peel it off.
Klaus