There are several things that you must not forget when doing this. Don't forget to put oil in the compressor, read the instructions for number of ozs. If it is a used one, then just replace the oil.
Change the rings of the compressor before putting it, because it can avoid leaks. When you put it in place.
Vacuum every line. Moisture in the system is a big problem. You can use some chemical cleaner and an air compressor for getting it out.
Replace the expansion valve (orifice tube).
Replace the accumulator, this is something you always want to do when you change you compressor.
Recharge the system using the low port. Before starting the car turn the a/c compressor pulley around 15 times to make the first cycle of circulation and grease your compressor, This step is crucial, if you don't want to start cutting your compressor life in a half.
I did this in my older car once, but I have never done it in my Focus and I hope not to do it. This and your manual should be good enough to do it.