see
http://www.generalfinishes.co.uk/ind...ge=preparation
tells you about sanding and the paper. As for the pressure start off with a little pressure see how it looks, you don't want to leave deep scratches in the wood (as would happen if it was coarse sandpaper) think of it as a nail file ;o) the coarser the file the more you chip the nails..lol.. only trying to simplify it a bit and that was the best comparison I could come up with.
I think a medium would feel twice as rough as an emery file.
Failing all that ask the shop asisstant, that's what they are there for.
Hope this helps