What is the basic technique for cooking amaranth?
The basic cooking technique is the same for whole amaranth as for many other whole grains (e.g., barley) and grain-like seeds (e.g., quinoa): bring water to a boil, add the grain, simmer uncovered on low until the grain has reached the consistency you desire (that is the only true definition of "done" -- never be afraid to taste your food as you go!), then let cool.  Even when packaging suggest soaking, I do not soak my grains and I have never had any problems.  OF course, others will insist soaking is necessary.  My advice, if you are concerned: try it both ways and see which you prefer. 

For each part of whole amaranth, here is the proportion of water:
-- porridge consistency, 3 parts water (some liquid will remain)
-- soft texture but with distinct grains, 2 parts water 
-- firm texture (pilaf), 1 part water

If the water has nearly all cooked off but you want a softer texture, add extra water and keep cooking.  If you are not sure what consistency you want, err on the firmer side, and taste as you go.  Note that larger batches (say, over 1/2-kilo of grain) need less water than smaller batches, so plan accordingly.

Adding salt to the water at the start is optional but I encourage it.  The point of adding it early is to incorporate the salt into the grains evenly.  Under-salting will mute the flavors substantially.  Adding salt at the last minute will provide a burst of salt on the tongue that is not always pleasant or desired, especially if you serve the grain with some other moderately salty food.

As for fat, I don't add any because I usually serve the amaranth with something that is already fatty (like an enriched sauce).  If you want to add a fat, then I would do so before the water has completely cooked off to ensure the fat coats the grains more or less evenly.

Oh, and I love using an automatic rice-cooker for my grains, as it automatically turns off when the temperature of the food rises substantially above 100 C (meaning all the water has cooked off).