The vintage way is:
Fix the cavity-plate with "glazing points"
in the cavity (those are sharp metal triangles, you press them in the wood, and secure them to the brass plate with a drip of solder.
If that's done, you place the 2 plates for the pickups (they don't need fixing, they're kept in place by the downward pressure of the pickups and foam), and connect the wires through the holes for the pickup-wires to the aforementioned cavity plate (just solder them to a glazing point that you've already secured). Next you run the earth wire from the cavity-plate to the bridge (on a non-shielded bass, this wire comes from the mantle of a potmeter), and you run a wire from the mantle of a potmeter (most likely the volume pot is uses) to the cavity-plate (to a glazing point).