Most cats do not need to be sedated for routine grooming. But cats rescued from dire situations may resist all efforts at grooming, nail trimming or bathing. They need sedation in order to have their coats worked on. A dirty, matted coat or a broken claw can bring the cat much discomfort. The cat will be more willing to be tamed if he is feeling healthy.
Go to a vet to get the proper sedatives. These are prescription-only medications. They will be in liquid or pill form. The usual sedative is diazepam (Valium or Diastat).
Place the cat in the towel or pillow case as best as you can and quickly wrap the cat so only the head is showing. Here, the helper will be useful to keep the cat from poking its forelegs out through the neck opening.
Place the cat on the floor and either have the helper straddle the cat or hold the cat in between your legs. Use gentle pressure with your legs just to keep the cat in place. Some people find that placing the cat on their laps works better.
Place a thumb and forefinger of one hand on either side of the cat's cheeks and apply gentle pressure. When the cat opens its mouth, take the other hand, press down on the cat's lower jaw and push the pill in. If the sedative is a liquid, make a pouch of one cheek and squeeze the medication in.
Let go of the cat's mouth and lift the upper jaw so the cat's mouth points to the sky. Gently rub the throat to encourage swallowing. Keep that position for a few seconds, then let go of the cat. Either unwrap the cat or let the cat free itself. Praise the cat and leave treats on the floor for the cat to eat at its leisure.
Work as quickly as possible in wrapping the cat and getting its mouth open. This is less stressful for the cat and for you. Expect to be scratched or bitten. If the cat panics or faints, then it is best to take the cat to a vet and let her sedate and groom the cat.
Never give a cat human medication. It will be far too powerful for a cat and probably kill it. Don't wear gloves because you will not be able to handle the pills.