Im intrigued as to why you'd want to do that tbh?
I wouldn't myself as the hassle in sorting materials for sites for effectively what the builders tell you your profit is going to be isn't worth it from my point of view.
Not saying you can't make money from it , just thats it's the long game for it to be worth while.
I can earn a decent living just by turning up on site and sticking them on tbh , whats not to like?
As regards doing it , you'd have to register as a contractor , produce H&S assessments , method statements , diffo insurance costs . Then of course when a subby says to an agent , nout to do with me m8 , the agent calls you . :0)
I'm lucky in that guy that I contract to is a pretty top bloke tbh , pays on time without fail , and is very accommodating to my health issues and lifestyle. He makes money , but he damn sure works all hours for it.
Just my view tho'.
Diggy