Slip the anchor ends between the seat and its back. Roll back the rear mat and choose a place that will allow the belt to extend straight down to the floor pan. Drill holes in the pan for the bolts. Don't mount the belts to the frame because in a crash, the body may separate from the frame causing the belts to become tight enough to cause injury. Place a large "fender washer" under the floor pan and over the bolt, and tighten firmly. Belts are always a good idea, and imperative in open cars. Buy the best belts you can find. That is no place to pinch pennies.