Assemble the upper and lower sides
Starting with a second stack of cut wood, fit, drill, stain and assemble the upper and lower sides to the metal frame. The lower sides are cut the 1/2 inch extra depth to accommodate the added width of the 1/4 angle iron. Also the lower sides have holes for the lift arm that comes later.
This a simple step too. Straight forward cuts, simple bolt and screw assembly. Take care. Everything else hangs from or bolts to these sides. It’s important to get everything level and straight. An error here is something you will have to work around or compensate for on the rest of the project. Take your time and you will be fine.