Be prepared to do the job twice if you don't know what you're doing. The biggest deal, by far, is flushing the old system. If you're replacing the compressor, you need to replace the drier and the expansion valve, you need to flush the crap out of the system, and really flush the crap out of the condensor.
If you don't do all this, there are high odds that you'll put the system back together, it'll work for a few months, then the compressor will fill up with residual crap and fail on you again.
Happened to me, and it's happened to a lot of people on this board and others.