As for the how, most pools have a website which tells you how, but in general you just follow these steps:
- Register on the pool's website and get your username/password
- Get the pools mining url (e.g. imaginaryminerpool.org:4321)
- Configure your miner with those options. E.g. in pooler's cpu miner, run it with the command line arguments below
minerd --user=username --pass=password --url=imaginaryminerpool.org:4321