Hardly the most complex of how to's this one It does surprise me the number of methods that people use, and most of them are not correct . . . . for brewing yeast! Bread yeasts are a completely different animal . . . actually they are not . . . and the requirements are different.
Firstly you need to use the correct amount of water, not wort or a sugar solution but normal tap water . . . preferrably water with a medium to high mineral content. The reason for this is simple, when you add a dehydrated yeast cell into a solution it rapidly absorbs liquid, but the yeast membrane is not selective at this stage and allows 'everything' and 'anything' across the membrane, including all those sugars and minerals. This can cause extreme osmotic stress to the poor yeast cell as it tries to wake up, and it will either end up poorly or could even explode and die, not a good result really, so just medium to high mineral content water.
Secondly the right amount of liquid should be used . . . Fermentis suggest using 10 times the weight of water as yeast, so an 11g sachet of yeast needs 110g of water (which is as near as dammit 110ml), 100g of yeast needs 1000ml
Thirdly, the water, after boiling to ensure sterilisation, needs to be at the correct temperature, and this varies for variety to variety. Nottingham for example needs to be at 36C . . . Fermentis Saflager W34/70 needs 21-25C, . . . SO4 needs 25-29C. Lallemand report a 20% loss in viability compared with rehydrating at 30C instead of 36C . . . and if you are close the limit for under pitching this could be an important factor. DO NOT guess the temperature! Use a thermometer! This is one of the easiest things to do, and guessing wrong will almost certainly kill your yeast.
Fourthly, Sprinkle the yeast onto the surface of the cooled water and stir gently to mix it in . . . I know it just says sprinkle it on and leave it . . . but you need to stir it to mix it in, (I actually use my stir plate while pouring the yeast into the sterile conical flask), and then leave it to stand (covered) to rehydrate usually 15-30 minutes depending on variety, (Lallemand say 15, Fermentis 30).
Fifthly, as you are filling the FV with cooled wort, give the yeast another stir, and pour it into the Fermenter.
Result, Happy yeast ready to get cracking to turn your wort into luscious beer.
should give you some rehydration temperatures to play with.