Look for a tiny valve you can turn on top - you'll probably need a radiator key to turn it, although a couple in my house were fitted with standard screws and all I needed was a screwdriver that could fit in that space. I can take a picture of ours when I get home if it'll help you. If you don't have any radiator keys, I BELIEVE they are universal and you can get them at any Menards or hardware store or what have you. Should be under a buck and last forever.
Here's what I did - there ARE good Google pages but it's probably better if I just tell you what I did.
1. Make sure there's water in the system - I kept having to add water to mine because the pressure would just disappear, there was so much air in the system. (My boiler is supposed to run at 28, I filled it to 25 and tried to have it down to 20 when I was done, since letting out the air lowers the pressure but heating up the water into steam RAISES the pressure - I have an overflow tank so I can be "off" but I err on the side of not quite enough water rather than too much water).
2. Go to the top floor and take a guess as to which one is furthest out along the line, and bleed it - turn your key until you hear air coming out - keep it open until the air stops coming out or you get water (have a cup ready to catch the water, dummy!). If you get water, fantastic - it's filled and you probably won't have to worry about it for the rest of this procedure.
3. Work your way back along the line from radiator to radiator until the top floor's done. I had four.
4. Go downstairs and check the pressure. You may need to go back to step 1, add more water, and repeat 2 and 3 until you get water from all of them.
5. Now go to the bottom floor and work your way back along the line with THOSE buggers. Again, you may need to stop, add water, and go back. Bottom floor radiators tend to give you water a lot faster, though. Another four done here.
6. One last check in the basement to make sure your gague says there's water in there.
7. Fire up the thermostat. Oh, I should have mentioned:
0. Turn off the heater before you start!!
Hopefully you're reading before doing. ;-)
The step I MISSED was:
8. Afterwards, check all the radiators in the place to make sure they're not leaking any water - or even so much water that it starts dripping from your kitchen ceiling and causing you to panic at 2am. The above radiator was the culprit, and turning that nut stopped the flow. Fortunately, I figured this out before paying a professional - what, $195? - to come on over and turn it for me!
Last day before the GREATEST SPECTACLE IN INDY 500 SOMETHING SOMETHING JIM NABORS STILL GIVES ME THE GOOSE PIMPLES so here's a tie to match. (The black flag is underneath the area you see here, so don't ask, funny guy.)