I did some research online about how to treat the surface panels and found a lot of different opinions (floor paint, specially made textured wall paint, etc.). I wanted some texture, but I went with standard exterior paint with some sand mixed in. I experimented to get the right amount of texture — just a small spoon of sand in a gallon of paint gave the right feel for me. Unfortunately, I did not record the number of plywood sheets that I used, but you should be able to compute that from your Sketchup plan. I laid out all of the panels and painted them on both sides with primer and my textured paint for weather proofing.
After the panels were painted, I went ahead with drilling holes for the T-nuts. There are some options for T-nut choices online. I went with the cheaper Zinc version over stainless steel (same for bolts for the holds). This is one decision that I might rethink — the steel versions were about 3x more expensive at the time I was building this, but some of the bolt ends and T-nuts are rusted now (5 years later), making some holds difficult or impossible to move. Find a spade bit for your drill that matches your T-nuts (and obviously the bolt sizes). I used the standard 3/8″ prong T-nuts. Look around — I checked a dozen or so sites and found one that was selling the same T-nuts as everyone else on sale for quite a bit cheaper.
I placed the plywood on 2″x4″s on the ground in stacks that were 3 sheets deep. I laid out a grid of 8″ squares on the top sheet using chalk lines. Then, I used the spade bit to drill holes at each intersection. Instead of drilling exactly on the grid point, I randomly offset each hole from the point by 0″ to 3″ to get some pseudo-random patterns while keeping good hold spacing. After drilling, the back of the bottom sheet splintered somewhat, but I cleaned that up as needed when I was pounding in the T-nuts later on.
Next (BEFORE you pound in T-nuts), I cut all of the panels on the ground, propped up on a few 2″x4″s with a skill saw, as in the photo above. That panel is a floor section for the fort, hence the notches for posts. After I cut the panels, I painted over the cut edges before screwing the panels to the wall to help prevent water damage. After 5 years, I have had to replace part of only 1 panel due to weathering, so all the painting was worth it! AFTER the panels were cut, I pounded in T-nuts from the back into all of the holes that weren’t too close to cut lines.