well, what kind of result do you want? if you want hot cereal, cook it 2:1, liquid to amaranth. bring to a boil, then reduce and simmer until it's cooked to your liking.
if you want to serve it as a side (like rice or couscous), try a ratio of 1:1. bring it to a boil, then reduce and simmer for 15 to 20 minutes. but keep an eye on it. i've had it turn from uncooked to blech in no time.