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.)