Why do you want to clean out just one radiator?
You'd be far better off to flush the whole system. Videos on YouTube which will show you how.
If you just want to flush one radiator and you've only basic tools. .....
Switch the boiler off, I'm presuming it's a combi. Turn off the valves at each end of the radiator then put a pretty big receptical under one end and unscrew that valve from the radiator. Then open the bleed valve at the top of the radiator and the water will drain out. Then disconnect the valve at the other end but block up both holes if you don't want mucky water on your carpet , then lift off the radiator from the wall and take it outside. You can then turn it upside down connect a hose to one end and pump water through it. Once it runs clear, turn it back to the right way up and you can pump out any muck that's left. Disconnect the hose and let it drain. Put a stopper back in each hole and replace the radiator on the wall. Remove the stoppers but don't be surprised if a bit of water comes out, so put some rags down.
Reconnect the valves and leave the bleed screw open. You will need to add more water to the system.
The boiler instructions should tell you how to do that. While it's filling get someone to keep an eye on the bleed valve to be ready to turn it off when water starts to come out after the radiator fills.
The boiler instructions should tell you what bar pressure you need the system set at. So keep letting water into it until it reaches the correct pressure.
Then turn on the boiler. It's likely that the water pressure will drop again so you will probably need to add more water to the system to get it back up again. I'd bleed all the other radiators whilst you're at it to make sure there's no air in the system. Let it run for a bit and then check the pressure again.