How should I calm my cat down?

I have two, large, aggressive, male cats and I have to take them to the vet, and it freaks out and scratches me everytime I try to put it in the carrier, and I googled seditives for cats and I got this medicine similar to Xanax called Alprazolam, and everytime I look up a website that says something about it and the proper dosage, it always tells me to give it to them by weight, age, health, etc. Well, my cats are young, healthy, and I'm not sure about their weight but I would say between 5-10 pounds, so what would be about the best amount to give them? I want something just to knock them out, not to put them in a coma or kill them.