Close off all the rads in the system on the thermostatic valve and outlet valve of the radiator.
Turn boiler off, crack the nut on the rad side of either of the 2 valves on the rad you want to remove (if you dont have a drain down valve) have a bucket to catch any water, remove bleed screw for faster draining.
Remove rad or whatever work you are doing.
Refit, tighten nuts.
Repressurise system, open rad inlet(thermostatic valve) handy to have a fitting on the bleed pipe if filling the system on your own with a hose into a bowl or bucket, otherwise you end up with water all over the floor.
Turn off filling loop
Refit bleed valve.
I normally add inhibitor to one of the towel rails in the system as they normally have a top mounted nut, rather than a side entry or having to use a special tool in the filling loop.
Always worth doing, as it stops the build up of sludge and scale and protects the boiler.
Start the boiler and check the rads all get warm after opening the valves, rebleed the removed rad.
Plenty of towels help aswell!