I have not done it myself, for our purposes it hasn't been something I felt the need to do, though I was tempted to. I was able to just process instead. I think it is a more practical thing to do for someone who isn't able to process at a moments notice or if the bird really needs more growing time.
The process of doing it is pretty basic. you need a large volume syringe (30-50cc) with a larger bore needle (14-18 gauge). alcohol or betadine to clean the area for the needle entry.
Hold the bird securely, pick an area midway between the point of the ribcage and vent, clean it very well, insert needle to just under skin, draw back on plunger to withdraw fluid. Do not move the needle around once you penetrated into the abdominal cavity, you don't want to risk lacerating internal organs, you want it to be as minimal an amount of trauma as possible. Most needles can be disconnected from syringes by twisting. If you have the bird held still and have someone helping you or are familiar with handling needles you should be able to disconnect the syringe when full without moving the needle, you can then empty the syringe and hook it back to the needle and pull any remaining fluid.
If you have never used a needle and syringe before you should get a couple of extra ones and practice with one by piercing a thinner skinned orange or the skin of a piece of chicken from the store. It will give you a bit of experience handling the syringe before using it on a live bird.
The process is explained on byc in a couple of threads on ascites I have found during research.
Edit to add...when you have finished draining the amount you wanted you will withdraw the syringe directly back out. The hole will be small enough it shouldn't bleed actively, though you may get a drop or two of blood or fluid. Wipe the area clean again and I would put a dab of antibiotic ointment on the site, and or a squirt of blu-kote. Treat the area like you would any other small puncture wound.
Edited by fisherlady - 10/14/15 at 9:27am