It depends on how nasty the rooster is, and how soon you need it caught.
If it runs straight at you when it sees you, just put on a heavy jacket and some safety goggles, and grab it when it comes for you.
In most cases it's easiest to grab a chicken when it's asleep in the dark, then put it in a dog crate or something similar until the next day so you can have light while you're killing it. To get the rooster out of the dog crate, just reach in (optional: wear gloves & long sleeves when grabbing it out of the dog crate).
If it's in a big pen and you need to catch it now, either use food to lure it into a smaller area, or chase it into a smaller area, or use a large fishing net.
If it's completely loose, try to get it into some kind of a pen first (lure with food, or chase it).
When you get it into a small enough area, just get in there with it, close the door behind you so it cannot get out, and grab the rooster.
If you're asking how to actually grab & hold the chicken, grab any part you can reach, then shift your grip to something secure that doesn't injure the chicken. I prefer to hold chickens with two hands around their body, or cradled against my chest, after I catch them. But I have caught plenty of chickens by first grabbing the body, the wing (usually near the shoulder), the leg, or the tail.