It needs an opening. You can lance it or soften a scab and pull it off.
It needs to drain. Gently press to encourage the puss to come out.
Flushing it with hydrogen peroxide is very effective, but quite harsh. I usually flush with peroxide a couple of times (once day one, and once day two), and use body-temp water or saline solution for all other flushes. Flush a couple of times a day for a few days, then once a day until it doesn't seem to need it any more.
You can use a small syringe (no needle) to get the peroxide, water or saline solution into the abcess, if the opening is small. Then gently press to express the liquid and all the ick it washes away.
You will need to keep it open and draining. Don't let it close up until the inside of the abcess heals. Do NOT put antibiotic ointment on it - this encourages the surface to heal over, which can trap the infection inside.
It has to heal from the INSIDE.
The cat may lose his hair in the area. That's OK, normal reaction to the infection and localized fever. The fur will regrow.
I always, always end our "doctoring" sessions with a treat for the cat, so he has some sort of reward. This, obviously, has nothing to do with treating the abcess, but makes the cat more prone to cooperating with Dr. Mom.
Keep an eye on him. Monitor his temp. Normal is in the range of 100.5 to 102.5. If it gets much higher, seek professional help. If he refuses to eat (especially if it's the "good stuff"), take him in. If he becomes lethargic, that's another danger sign.
If the cat is hard to handle while you doctor on him, have an assistant hold him, and/or wrap him in a towel.
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