Put a cup of water in a very clean bucket, and mix in the powder slowley. Keep beating it up like you would an egg and make sure if anything while you're mixing it, it seems too dry (all of a sudden grout usually just seems to go to a paste when 3 seconds ago it looked like it needed more water). You can always add more so never go to heavy on either grout or water.
What you want to end up with is a mixture that would not fall off a spoon if you held it upside down, but it is soft enough for you to push into grout joints.
Only with certain coloured grouts have I ever bothered to weigh it out. And that's only because the colour can change slightly if you have two mixes with totally different consistances.
What colour is it by the way? And is it for walls or floors?