Put the pillar base on the knee wall and make sure it's level (Image 1). Use shims to correct the level, if necessary. Remove the base and apply construction adhesive to the knee wall. Re-insert the base and re-shim, if necessary. Nail the base into place using the nail gun (Image 2).
Cut two square blocks of wood that will fit into the interior of the column. This will be the extra support to hold the four sides of the column together. Lay one of the ripped 1x6 pieces face down. Put a square interior block on top of the end of the 1x6 piece. Measure out to the height of the column, and place the second square block at that measurement. Put another piece of ripped 1x6 on top of the two square blocks. Use a speed square to make sure the interior blocks are square (Image 1). Nail the top 1x6 piece into the interior square (Image 2).
Mark the column at its intended length (Image 1). Using a speed square, mark this length off on all four sides of the column. Using a circular saw or chop saw, cut through the column one side at a time (Image 2).
Place column on base (Image 1). Make sure it's level, and shim it, if necessary (Image 2). Toenail it into place using 16-gauge nails (Image 3). Apply crown molding to the top, if desired, and stain or paint.