3-point seat belt install help

[Breadstick]

I cannot be this stupid. I got the 3-point seat belt and the mounting plates for my 64 C10 from So-Cal and they finally came in. I was looking at them and doing some dry fitting and I can't figure out how they should mount. I know it's much simpler than I'm making it but I have just spent an hour going over every way to make it work and I'm coming up with nothing. I can't find anything with good pictures to show me what to do anywhere.

Please help me.