Baking Projected Textures

I need a Tutorial on baking textures into UV space.
Sometimes I can get the Open GL UV render to do it, sometimes it wont.
I've never got it to do it by purpose .

It seems like i'm missing something or buggy?
Any help would be great!!!!

Thanks so much

could you provide an example comp which fails? because right now is very vague to figure out your problem without knowing what you're doing