I use a 2 foot long (~60 cm) spirit aluminum spirit level to do mine. I have done them all with 320 grit double sticky taped to the bottom. Would probably take me less time if I used the grit Simon does.

Here in the US at 2 foot spirit level is a standard size so it's readily available, and I like the length. long enough to keep things really flat, but short enough that I can get some decent strokes in without smacking into the nut. (I don't have Simon's finesse ;-)

Everything else just like Simon said. The one thing I'd add is not to bear down on the beam as you sand. Just the weight of the level is enough and helps keep it completely flat.