Assumptions at this point:

1. The platform has been completed.

Description

The body is also the frame (a "unibody" structure is the automotive design term). This was simplest, strongest and easiest to build than other options such as body-on-frame. The sides, and rear divider are attached to the above platform and firewall with screws. The tailgate and frunk are then cut, assembled and attached. The cowl is then assembled and attached using a hinge attached to the firewall. Steps:

1.  As indicated by this "assembled vehicle" 3D diagram in OnShape, the sides are screwed to the platform and firewall using 2" flathead, square hole screws. As a rough guide, these should be about 9" apart using a pilot hole and countersink.  (Of course when attaching the 9" sides to the firewall will require an exception. In this case 2 screws 7" apart will be adequate.  Always trial a full attachment between 2 scraps to be sure of hole size, depth, strength... Then trial each screw placement first to check if: 

- there is enough material (plywood) to accept the screw 

- if screw will be sufficient distance from an edge of the plywood to prevent ply separation  (~1 inch minimum)

-  if there will be interference from other installed screws, bolts, attachments... etc. 

When drilling a pilot hole, keep it perpendicular and drill to a depth that leaves a ~ 3/4" penetration of the screw into solid plywood (no hole). 

This is a tedious process so use 3 drills if you can - 1 for the pilot hole, 1 for the countersink, 1 to screw in the screw.  Use a manual screwdriver only to test the first screw for fit, depth, strength, etc. Any more than that and you will develop blisters and carpel tunnels.  If you have only 1 drill, use quick change bits to quickly change between drill bit, countersink and  square screwdriver bit.