As a DIY'er, I remove the Thermostat and screw a cap on there (to close that valve off. Then I close the Lockshield valve. Now the rad is isolated from the rest of the system.

I then place blankets on the floor and undo the Nut on one end of the rad, draining the water into a paint tray (empty it into a bucket when it gets full), and so on.

Then, once most of the water is gone, you can put your thumb over that end of the rad, before undoing the other end. If you put a tray under the other end before you undo it, and tip that rad up you can avoid losing the rest of the water, before carrying it outside.