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.
Net works, but easiest way is to chase it into an enclosed run where you can more easily catch it. Two long sticks make it much easier to 'herd' the chicken where you want it to go. Once it's in the enclosed run, it's easiest to chase it down by standing relatively back and using the fact that chickens will change direction against them. By using your arms spread wide you can get the chicken to almost spin in circles and so approach quickly. Then, easiest is to get a firm hold of the feet. If the chicken is panicked you can hold it upside down and away from your body until it tires out which usually causes less injury than trying to grab ahold of it while it is flapping. Put it back upright and then hold the wings close to its body like you would lifting a hen.
Aww poor guy! (Excuse my heart for roos.)
Have you tried the discipline method? I'd hate for him to die without one last chance!
Basically you catch him after an attack, sit him down in the dirt, (preferably in front of his ladies (if he has any)), and put his beak in the dirt for about 30 seconds. This humiliates him in front of the flock, and he'll usually keep away from you after this. My roo charged at my sister one day, and I found this online, and tried it on him. He never attacked us again.
It's worth a shot! Don't kill him yet!!!😭