How do I use Extensions

I’m trying to find how to implement glLockArraysEXT but I cant figure out how to get my app to compile with it. Do I need to add something to gl.h? I’ve looked at the document here
but that wasnt clear. I need a step by step guide, Im not bothered about detecting whether my card can handle it, I know it can, I just want to use it. Looking at the document above I think I have to add something like typedef void (APIENTRY * PFNGLLOCKARRAYVEXTPROC)(GLint first, GLint count);? Also there is very little documentation on glLockArraysEXT searching on it returned nothing, anyone got any more info on it. Thanks.