Read the directions on your plaster to find the correct proportions of water. There's a fair amount of leeway here; more water makes a thinner slurry that takes a bit longer to set, but pours more cleanly and has fewer bubbles.
Estimate the amount of plaster you'll need by eye. The plaster sets much slower than the alginate, so if you don't have enough, there will be time to mix up a bit more. (You can even pour more plaster over cured plaster, although the color won't match exactly and you'll see a line.)
Again it is powder and water, but this time mix the water into the powder. Use a large bucket where you're unlikely to spill. If you have a power drill mixing attachment that works great, otherwise mix vigorously with a paint stirrer or other hefty stick. Scrape the sides, bottom, and corners frequently.
Here's what the mixing looks like:
(I unfortunately did not track the proportions we mixed, but I believe it was about 300 ml of plaster with about 150 ml of water.)