I learned several things the first time around so while some of this is a rehash, some of it is new.
First things first you will need a KX450F rear brake caliper... Any year will do up to at least 2012
Next, strip it bare..
{TIP} Use a air nozzle to blow the seals out... Most of the time you can re-use them.
Or just buy new ones as they are cheap..