2: Put them in a bowl and cover with hot water (not boiling). There should be about 1/2 an inch (1 cm) between the floating mushroms and the bottom of the bowl. Cover and leave to soak for half an hour.
4: If you are using morels, which tend to be full of dirt (and only in this case) put them in a fresh bowl of hot water and leave to soak again for half an hour. For other mushrooms, one soaking is enough.
5: Meanwhile, go back to the soaking water which is much too good to throw away. It has been transformed by absorbing the flavour of the mushrooms and is now a very flavoursome mushroom "jus". Scoop out this "jus" with a ladle, being careful not to tip or stir up the water. Collect as much as you can that is "clear", without any dirt or sediment.
6: Keep the mushroom "jus" in a sealed container in the fridge, but for a few days at most. It can be used in any savoury recipe that asks for a flavoured liquid, such as stock (like morel risotto with vin jaune and mont d'or).
7: The final thing to do with the mushrooms is to drain them thoroughly before using. The best way of doing this is to use a salad spinner. Once drained, your rehydrated mushrooms are ready for use.
Remarks
With morels, the proportion of dried mushrooms to rehydrated is 1:3. In other words, 50 g of dried morels will give you three times that weight when reyhdrated: 150 g.