Go to a vet..if you choose not to, then know this:
- a board/hammer won't work. It's like trying to smash a baseball..the skull is very hard
- muscle relaxers. This could slow down and make it better, but cat will still be awake and aware
- oxycontin. This could work
- Asprin, not sure, this could work
- bullet in the head. Problem is the head is small, you could easily shoot your hand
- sleeping pills, this could work and might be best since cat will be asleep
**with any of the above pills/meds, know that you will have to spend a few hours observing the cat, it's a long haul.
Go to a vet. Vet's charge a lot, but this is because the process requires needles, meds, and a vet's time.
A veterinarian is your best bet Im afraid. I tried to put down a 8 year old cat whod been hit by a car when I was a teenager using a horse tranquilizer. It was a stray Id been feeding. Not only did I fail, he was a monster after that. He was insane and lived 15 more years. I got him properly vetted after a few years once I had a proper job and they said he was just a mean cat. But I knew it was my fault. He finally got peace at 23 years old. Id never recommend trying to euthanize a cat yourself.