http://www.cleanitup.co.uk/smf/index.php?topic=129758.0There are pics in the thread above showing how I did mine.. I chose not to use a hole saw as it seemed a bit imprecise and I wanted to cut the hole right tight so the immersion would cut its own thread as I screwed it in. Still going strong. (I used a dremel type tool after marking the hole too small deliberately and then gradually making it bigger until it was just right).
I've just done one with a mechanical flange for a friend (google "mechanical flange immersion", you'll see what it is), but they're increasingly difficult to find now, old tech.. Toolstation used to have em but not any more. I got mine from Ebay for twice the price, and they've gone up another two pounds since I bought mine just a couple weeks ago. The only caveat I have about using a mechanical flange is that the standard wydale tanks that I have experience with are just a BIT too thick to use with the flange, you have to mod the flange and it's a pain to get it tight, but it is certainly do-able.
good luck!