Watch this video from This Old House to learn how to install a radiator thermostat.
1 Close the valve at base of radiator.
2 Turn off the room's thermostat.
3 Carefully unscrew the existing air vent from side of radiator.
4 Put Teflon tape on threads of new thermostatic steam valve.
5 Thread new steam valve into radiator; tighten with adjustable wrench.
6 Apply Teflon tape to threads of old air vent.
7 Thread old air vent into top of new steam valve.
8 Open valve at base of radiator.
9 Turn on the room's thermostat.