Okay, let's layup the firewall.
I like to add some .030 carbon plate to the firewall, it looks super cool, adds little weight and just makes the final product look more professional. I get my carbon plate from CST -
http://www.cstsales.com/carbon_plate.html
What I do is prep the firewall ply and balsa pieces with some 150 grit sandpaper. Cut a piece of the carbon plate about an inch bigger than the firewall. Rough up one side of the carbon plate with 150 grit and cover the other side with masking tape or a product like Sticky Mikeys frisket material. You can get it here and it's handy stuff -
http://www.dixieart.com/Accessories.html
At this point you need to identify what is the front of the firewall, the plywood donut is shifted to the left while viewing the firewall from the rear. The balsa firewall doubler goes behind the firewall, with the carbon plate in front.
Once everything is prepped, I mix up a small batch of West Systems epoxy with either the 206 or 205 hardener -
http://www.cstsales.com/west_system_epoxy.html . I coat the front of the ply firewall and place it on top of the prepped side of the carbon plate, I then coat the backside of the ply firewall and place the balsa doubler in place - I then run a bead of epoxy around the edge of the ply donut and slide that into place. Wipe any excess epoxy away.
I lay a piece of wax paper onto of the stack and put 150-200 lbs of weight ontop while it cures overnight.