Cheaper to buy them seperately (if you can find them), more convenient to buy premade - the formulas will vary by manufacturer/product. For example, the ultimate burn has 334mg mahuang (ephedrine), 100mg caffeine, 25mg white willow bark (asprin) per pill. It's recommended that you take 1 pill twice daily 30 min before meals or exercise. I personally take 2 twice a day, but definitely start out at not more than the recommended dose, and really maybe less - it can hit you hard!
Some people take it only pre-workout just for extra energy. If you are alternating with clen, you can go 2 weeks clen, 2 weeks eca, 2 weeks clen.....
In general, don't stay on ECA for more than 30 days straight. 2 weeks on, 2 off seems to work well for it imo.