How Do I Drain This Boiler?

Purchased a house with a one-pipe steam system fed by a gas boiler. There is a hot water baseboard radiator loop coming off the boiler to heat the basement (separate thermostat, have never turned the basement heat on).



A plumber -- not the one who installed the boiler, that was done under the previous owner -- said that I should drain the HW tank every three months and the boiler every month. How to do the former is obvious; I'm a bit confused about the latter.



One one side of the boiler are two spigots with a pump in the middle -- the hot water pipe. If I open up either spigot I get rusty water coming out. On the other side of the boiler is another pipe, closer to the floor, with a spigot and an additional shut off valve (pipe is partly insulated and runs between two bricks in photo). I haven't touched the spigot on this side.



Should I drain the boiler via the hot water loop? If so, does it matter which spigot? Or is there something I need to open up at the bottom of the boiler itself, behind the access panel, in order to drain it? Finally, how much water should I drain out -- should I let it run until no more water else comes out? Is there something else that needs to be done to get the gunk out of the very bottom of the boiler?



Given what I've discovered so far (very rusty water in the gauge glass, pressuretrol set to 2.5-3.5, lots of hammering in the radiators, etc.), I don't trust the previous owner to have performed basic upkeep on the boiler, so I'm trying to play catch up.