Look at the back of the compressor. You will see three 19mm hex head fasteners. Loosen all three of them. I have found that a long 19mm box end works good. You can get to the two bottom ones from under the car and should be able to get to the uppermost one from above the car. You will also see a 15mm hex head fastener holding the hose "manifold" to the compressor with another 13mm hex fastener supporting some of the weight of the manifold. Loosen these before you get the 19mm fasteners out. Now that you have started loosening the 19mm fasteners you will want to back the belt tensioner fastener that pulls the compressor to put tension on the v-belt. Loosen this one pretty much all the way. Once you have backed it off you should be able to remove the v-belt. Go ahead and finish removing the hose manifold fasteners and then take out the 19mm fasteners. Capture and remember where the different size spacers go. You should now be able to remove the compressor from underneath the car. It does have a little weight to it so you will want to be holding it securely when you remove the last 19mm bolt. Pay attention to what kinds of gaskets are between the hose manifold and the compressor. You will want to put new ones here. Your new compressor may have a different type so you will want to make sure you use the right ones. You are going to want to get approximately 2 ounces of oil in the compressor. The way I do it is to lay the compressor face down, pour oil in both of the holes and turn the compressor where the internals are turning. Add some more oil and repeat. Do this a couple/few times until you have put about 2 ounces of oil in it. If you overfill during this procedure it will just spill out a little when you bolt it up. Anyway start putting everything back together. Position the compressor into place and try to get the 19mm fasteners started. When you have them going you should be able to bolt up the hose manifold. Don't forget to put the seals in place and I always use some Nylog on the seals. Tighten the hose manifold fasteners and then slip the v-belt over the pulleys, pull it tight and tighen everything down. If you are just doing your compressor you will want to measure the amount of oil that you pour out of it and then just add the same amount. Getting the correct amount of oil in there may be the hardest part of your whole job.
Are you going to vacuum/charge the system yourself or have someone do it? Are you sticking with R12 or going to 134?
good luck

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Jim