Follow the direction on the box or bag. Really.
I usually use 1 cup dry quinoa (and I don’t rinse it) to 1 1/2-3 cups liquid. I like using water, then I can mix it with other things, but I’ve also used chicken broth.
If this makes too much, dump the cooked quinoa into a ziploc bag and freeze it. To use, put it in your microwave on a defrost setting, just until you can mush it between your fingers.
If you want to get fancy, you can go this way:
Wash 2 cups dry quinoa (about 10 ounces) in 3 changes of cold water in a bowl, draining in a large sieve each time. Cook quinoa in a 4- to 5-quart pot of boiling salted water, uncovered, until almost tender, about 10 minutes. Drain in sieve, then set sieve over same pot with 1 inch of simmering water (water should not touch bottom of sieve). Cover quinoa with a folded kitchen towel, then cover sieve with a lid (don’t worry if lid doesn’t fit tightly) and steam until quinoa is tender, fluffy, and dry, about 5 minutes. Remove from heat and let stand (still covered) 5 minutes.