Turn off valves at both ends of rad. if you have a thermostatic valve one end then take the head off and fit the decorators cap to turn it off if you have one.
At the locksheild end try to count the turns it takes to close it completely.
Once both valves closed put down plastic sheet and/or dust sheet under valve nut. Put small plastic tray under valve nut. Undo valve nut between valve and rad slowly until water runs out. When water stops open air bleed on rad to let more water drain out.
When water stops running out lift one end of rad and drop over empty plastic tray to empty last bits of water, careful here as this will probably be black and/or rusty water.
When finished draining do up air bleed, undo other end valve nut, remove rad and turn upside down to eliminate drips from valve bodies.
Fitting is a reversal of above except that after fitting you will have to top up system pressure via combi filling loop or filling link built into your boiler.