The Brooklands screens are "universal" and so no instructions are included. Your imagination provides the guidance for mounting method and location. The wipers will be useless once you remove the proper windscreen on your MGA. So will the top and sidecurtains, which need the windscreen for support and location.
Personally, I don't favor the Brooklands screens on an MGA. It's just a matter of taste and preference, but to me, they are from an era just past as the A was coming to the fore. I'd prefer a cut-down stock screen, or a fabricated low curved screen made from Lexan. Either of these is quite a project, and they ALSO make the use of sidecurtains and top impossible. I guess I haven't helped much, have I?
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon