It's pretty easy -- fun actually, especially if they all come apart. It's a jigsaw puzzle putting it all back together, but just that easy and fun. Cassette tool in lockring, chainwhip to keep the cassette from spinning, and a wrench to loosen it. Loosen the lockring all the way, and try to life the rings all together. Put the new one on, making sure the spacers are all in the right place and it's all nested together. Tighten, and off you go!
For 3 days, I get to part of a thousand other journeys.