Here's a basic 3 step guide on how to build your frame for your kit and fix it to your kart. Feel free to change the design or dimensions to what suits your kart as this is an example of what works.

Step 1.
Line your side board up focusing on the wheel arch being centered around the rear tire and drill 3 holes though the side board and the sidepod. Holes should be sized according to fixing diameter. Recommended Dzus fasteners here.

Now fix the sideboards to the sidepods.

Step 2.
Cut 2x aluminium angles (at least 20mm x 20mm x 1.6mm) to desired length and attach to inside of both sideboards. Wide head rivets or small countersunk bolts with washers can be used.

If your sidepods are curved you may need to cut notches in the angle so it bends and conforms to the pod.  If so, rivet a piece of flat bar over the notched part of the angle to keep it rigid.

Step 3.
Build the rear wing frame from aluminium angle to fit in the width between the side boards and then bolt the rear wing panel to the frame as you bolt the frame to the side board supports.