Is there a proper mixing of plaster of paris and water? How much water will I add for e.g. 5 spoons of plaster?

How long does it dry up?

Plaster of paris (POP) should be mixed to the consistancy of heavy cream. The amount of water needed will depend on the ambient humidity of your area. More humid, more water etc. You should be able to vibrate out most of the air bubbles before you pour. Too thin, and the plaster will be light, and weak. Too thick, and the plaster will be dense and difficult to carve. POP sets up quickly, but give it 24 hours to dry completely.

Experimentation and experience will be your guide.

Are you going to pour it? If you're not, save yourself the hassle and use spackle instead. Already mixed for you in the little tub and it will the shelf life is almost forever...

Something I learnt from a "brickie" and never betrayed me: just put the plaster powder on the bottom of a pot, cover it with water avoiding to stir it if possible and let it rest until no more bubbles come up then get it with a spoon and put it in the mold...
Ciao