John the only reason I mentioned felt etc was in case you thought it may be an idea to put it under the Extratherm to just take out a few bumps, I know you never mentioned it
As for the self levelling, mix as per the instructions, an electric mixer is easiest, one of those paint stirring attachments on an electric drill works fine if you keep the speed down.
As for putting it down it is very hard to get a perfect finish, it does self level but not a lot and don't be tempted to keep trowelling it, get it as good as you can then leave it.
I find it best to trowel a very thin covering then, while it is still wet, put a thicker coat on. As I said it doesn't need to be perfect
Keith