Okay, so my running of Paizo's Shackled City Adventure Path has reached the final climactic end battle. I experimented earlier with making battleboards using foam-core poster boards and decided I'd do a big one to make this battle more of an event.
First up I took a close look at the Chamber of the Tree in the Fiery Sanctum where the baddies constructed their big portal opening device. At 1 inch (25mm) to square on the map it would just fit on a 20" by 30" (500mm x 750mm) poster board.
I got some wrapping paper with 1" (25mm) squares printed on the back and taped it over the board. Then I used that grid to help free hand draw the main outlines of the cavern walls and lava channels. When that was done I used a craft knife to cut through the paper along the traced lines to mark the actual board. Removing the now shredded paper I went over the cut lines in the board with black felt tip marker to outline everything. Next up came yellow fill with red and orange swirls for the lava channels.
For the lava rock floor I took the opportunity to use up some old gray acrylic paint I never really liked the shade of anyway.