Compression check on an out-of-body engine?

Ok, well I dunno if that's the correct term for the engine but it's not in any car right now but I want to do a comp. check on it before I buy it, it's at the breakers right now. I know I have to rotate the crank to check it but how exactly do I rotate the crank? I have some info on how to do comp. checks from a website, I just need to know how to do it on an engine sitting on the ground/shelf/whatever. Details would be appreciated, cheers, G :)

use compressed air.

get a nozzle with gauge on an air compressor to fit the spark plug holes and set the cylinder to be checked to TDC... then open up the air valve and read the gauge.

basically a simple leakdown check to make sure all the cylinders hold the same amount of pressure.