Step 1: Thrust chin forwards.
Step 2: Tongue behind lower lip but over teeth.
Step 3: Try to say, as loud as you can, with your face in this position, BELM! BELM! It should come out as an obtrusive grunting, keening noise.
Congratulations! You have belmed!
Sometimes referred to as doing a Joey, after Joey Deacon, belming is, to British people, especially kids, the universal shorthand for if someone is acting in a clumsy or clueless or similar fashion. Someone falls over their own feet at football and lets in a really easy shot like how Robert Green did at the 2006 World Cup? Belm at them! Someone answers the question, "Where does curry come from?" with "Jersey?" Belm at them! It's not very nice or very politically correct, but how else are you going to express your derision at their abject failure? Especially when you are insufficiently mature to understand very much?
I suppose I should explain a bit about where it comes from. Joey Deacon, that's where. In the early 1980s, he appeared on Blue Peter on BBC in an attempt to persuade the children of Britain to be aware about mental disabilities, namely cerebral palsy, and that despite this disability, it did not mean that they were incapable or inferior or broken, just that it was something that had to be worked around. A noble effort, I'm sure you'll agree. Unfortunately, kids being what they are (i.e. evil little fucks), they found his face, which had a constant expression, unfortunately, as if he had stuck his tongue under his bottom lip, and his voice, which was impenetrable other than by his similarly disabled friends, to be sources of comedy. So anyone who did anything "mongish," as the term might have been at the time, was clearly a "Joey." And over time, the fact that the sounds that did emerge from him sounded, to most children, like "BELM! BELM!" it was clear that what people who attempted to imitate him did was, of course, belming.
So now you know.