No you do not have to pump anything out... If the system is completely drained it will require a jumper wire to even get the compressor to work.
I am guess here without breaking out the manual, but lets say it is required to be 30psi... and yours is at 25, just start the vehicle with the A/C on, attach the gauge get a reading in this case it will show 25, then with the refill tank attached pull the trigger until it reaches 30. Then if you have rear A/C, turn it on and get a reading... for the sake of this we will say it is to be 38 and you are now at 35, fill until it reaches 38 and you are done. Recheck in a week or two if it drains off you have a leak. If not you are good to go.