would like to know the best way to drain ac sys to change the compresser

you need a recovery machine and ability to vacuum it out when done. you also need a dryer and an expansion valve if the compressor locked up. Roy

In addition to the first comment, if the compressor is noisy or locked up, you will need to flush the system and install a filter on the low pressure side before the compressor to prevent debris in the system from damaging the new compressor.