Well, I gave it a go. I don't have a water changer so went for the old-fashioned Jugs, Bucket and Swearing technique! After feeding my fish I switched everything off, emptied 20% into the bucket I had bought just for PWC's, emptied it and then did as some people suggested - mixed hot and cold water from the tap until it reached 24 degrees, which was probably the most annoying part. I took a PH test and it read 7.2, which I thought would be ok to add to the tank straightaway. I then mixed some Dechlor in and left it to settle for a few minutes before adding it all back to the tank. The only thing that went really wrong was that when I added it back I didn't do it gently enough and the sand at the bottom of the tank splashed up a bit, but that settled back down after a few minutes.
Having read that, how do you think I did? Any pointers for next time?