To properly evacuate the freon, you need a machine that pumps the freon out of your A/C system and recycles it for future use. After that is done, you can replace the A/C compressor. I would also recommend flushing out the lines and replacing the dryer (especially if your compressor failed and put some debris into your system). After repairs are completed, the refrigerant recycling machine is again used to remove all of the air from your A/C system, then it pumps the correct amount of freon & oil back into the system.
If your system already has a leak, then there is no freon left in the system to pump out. You can install your new compressor and proceed with the evacuation & recharge steps.
If you don't have access to an evacuation pump, then you should take your vehicle to a qualified A/C shop to have that step completed.
Hope this helps,