On a trampoline sheeted main, you normally have a purchase system at the back of the boom to the traveler (8:1, 9:1 or similar). The sheet line goes from one of the top blocks forward to a block in the middle of the boom. It then goes down to a block mounted on the trampoline and you sheet from there.

Personally, I don't like the trampoline mounted block. I think it makes the mainsheet spongy and reduces feedback. I had a trampoline mounted sheet but I removed the trampoline hardware and reconfigured the boom so the ratchet block was at the middle of the boom. I like it there much better.

Keep in mind, you need a substantial boom to support center sheeting.