Longboard center fin box install 2+1

Hello,

I'm doing one of my first boards, with a 2+1 fin system, and I'm not sure how to put the longboard 10.5 center box on the board.

I was wondering if you can help me with:

- When do I put the center box?

- How do I put it? (My box came with a top edge, which I will sand, and some foam stickers);

I post a picture with the material I have.

I read some topics here, there are several with information that I thought good, but I couldn't reach a conclusion.

Thanks