Simon,
It's just like washing a car, lots of hot water and soap. You need some long thin brushes to go through the
oil passages. The most important thing is to get all the little bits of metal out of the
oil passages. As soon as you're finished, dry it with paper towels or clean rags (in the sun, if that's possible). Spray the bores with some light
oil like WD40 to prevent rust, then paint it.
It's pretty easy,
Paul