But my question is what you do to complete the mould, do you put anything on the back of the mould to ensure that there is no sharpe dges to puncture the vacuum bag?

Breather works too, but I only ever use it if I'm double bagging and the stuff isn't going to get saturated with resin. Not only does breather soaked with cured resin make your mould a real pain in the ass to use; it can also destroy the part you're doing if it saps out more resin than you can afford, leaving dry spots. Which is easily done.