hi ryan.
first, you need to drain it and take it out. if you ahve a look underneath the radiator you will see a small screw, if you undo this the water/coolant will come out. do it on the grass
afteryou ahve the radiator out,you need to reverse flush it. this means basically letting water flow through the way its not normally flowing.
you can get chemicals to assist you i think, but usually just a reverse flush is fine.
remeber when you put the rad back in to fix all the hose clamps so you can unscrew them easily next time! (ie all the screws face up)
and dont forget to put the screw back in the bottom!