Harness mounting is fairly specific, best to read the instructions that are for the actual belt you're installing. But for general information, the Schroth book is pretty good. Regardless, seats require a proper anti-submarine slot. It's very dangerous to use a seat that doesn't have a slot.
https://www.tillett.co.uk/shop/docum...ation-2012.pdf
5pt belts are typically done with the anti-submarine strap following the chest-torso line and then mounted between that line dropped down to the floor, or 20deg forward.
6pt belts are either run through anti-submarine straps, through the side slots and you sit on the anti-submarine belts, or under a thin panel in the seat. The mounting point is quite specific to the belt you're using.
-Dave