Firstly, switch off the radiator for safety reasons before beginning to clean the inside.
You can fashion yourself a cleaning device for the inside of a radiator by using a thin wooden stick or plank with a towel or cloth attached to the end. This will allow you to access small and narrow crevasses within the radiator, and eradicate any dust and dirt that could cause blockages.
To find out more, visit our dedicated blog, How To Clean A Radiator.