You want to ensure you shut the water off to the autofeed valve, assuming you have one. How much you drain depends on the height of the room you are dealing with and if there's anything above it as there might be some leakage from other zones. Assuming you have a good air extraction device, and how the pipes run, you may not have to do much more than turn the autofill valve back with the bleed valves open until you get water, then shut off the bleed valves and turn on and activate the pumps. If you can do that without firing the boiler, that works too. Run it long enough to get as much spitting as you can then close the bleed valves. If the system was designed well, that, along with the air extraction, will clear out the air.
Once you get most of the air out, fire up the boiler and check for heat at each radiator. You might have to rebleed things a couple times, as an air lock might prevent water from circulating.
See what the pros have to say...but, each system is a little different, and may have its own quirks depending on the layout and installation of the piping.