The 'correct' way (Dental training!!) is to add the plaster to the water..NOT the other way round...
Coloured water with dies or as Ian told you with tea is OK but DO NOT use paint as it acts to speed up the seting time .(you can use it if you wish but dont mix too much at once)
Add the plaster by sprinkling it onto the water ...keep doing this untill the surface plaster has a very thin dry surface...leave for 1 min then mix by folding the mix together...
To Slow the set you can use Ice cold water or a tiny bit of washing up liquid...
To SPEED up the set time add a pintch of salt to the water before you add the plaster..
There is no ratio that is correct...it depends on the way the plaster was made and its quality.
Alasdair