How to pay CGT?

Sorry, this is probably a stupid question, but I've never been so lucky to be liable for CGT, so I haven't got the first clue.

I bought shares and made some profit. I am just not sure how to pay the CGT now? Can I do this immediately or do I have to wait till the end of the year? Is there a certain form I need to fill in?
Sorry for the stupid questions, all I know is that I have to pay it, I just don't know how and when!!!

See .

Ah, very useful, many thanks! Loving the fact that the first 1,270 are exempt, I did not know that!!!