Firstly, I think they make a chemical made to clean out the block's cooling passages. With that being said:
Disconnect rad hoses,
Remove rad (if you want to get it rodded out or something)
Coolant will come rushing out
Stick a waterhose in the lower hole and let it get what's left out.
You can either put it all back together with new coolant, or if it has a lot of dirt/rust/corroding or whatever, you can put some stuff in there designed to clean it out, and drive around for a bit like that (instructions should be on the bottle) and then you put regular coolant in.
IN the truck shop I used to work at - we'd use Cascade detegent to flush cooling systems. Run them for an hour or so - then drain them. Shoot water through the upper inlet on the radiator - and if your water pump is off, shoot water through both ports on the engine.
I'd use probably a cup or so with a car cooling system (I am talking of the powder detergent). Run that with water and drain it. Do that till you think its clean. Then run straight water through it till it comes out clean. Its a little time consuming but it seems to work well - better than those bottles of radiator flush.