Purchase a building permit in your jurisdiction prior to beginning construction to prevent being cited by a home inspector. Disregard this step if your jurisdiction does not require a permit.
Measure the total square footage of the concrete slab by measuring the length and width of the perimeter and multiplying the resulting numbers together. Measure inward about 10 to 12 inches from the perimeter and mark the area.
Repeat for the three remaining sides. This newly marked area is the actual area for the oven, as the 10- to 12-inch footer is meant to add stability and provide a buffer area to keep possible embers from falling into the soil.
Hammer stakes into the ground near each corner of the slab and tie a chalk line between two of the stakes. This provides a temporary guideline to keep the first line, or course, of blocks straight.
Mix the quick drying cement as directed and use the trowel to shovel a generous line of cement on the slab, keeping in line with the chalk line.
Remove the stakes and lay the first cinder block in the course. Tap it down with the butt end of the trowel, scrape some more cement on the end of the block and lay the second block, pushing its end flush to the end of the first block.
Tap it down and scrape away the excess cement that bulges out from between the joint. Repeat for three sides to complete the first course, leaving one side open.
Lay the second course in the same manner as you did the first, staggering the blocks on the second course so that they lie over the joint between blocks on the bottom course. Finish all the courses until all three walls are roughly shoulder height.
Allow the cement to cure overnight. Follow the previous steps to lay firebricks over the interior of the block walls in the same manner as you did with the cinder blocks.
Firebricks are highly resistant to fire and will prevent damage to the cinder blocks. Allow the cement to dry.
Select a spot on the interior walls in a joint between courses about waist height. Drill a small hole in the selected spot and drill another one on the wall directly across from it using an electric drill outfitted with a masonry bit.
Repeat to make another set of holes and insert the grill into the holes. Finish the wall as you did with the other three walls, leaving space for the oven door that you add according to the manufacturer’s directions.