The only thing tricky about adding arms to the chair is the mounting itself. Each side of the chair will be taking 5 screws 5" apart, exactly centered in the top edge, which is ½" wide. If these holes aren’t drilled properly, the drill bit could break out the side.
You can control your drill bit better if you make a drill bit guide. It can be a piece of sheet metal, bent at a right angle or attached to a wood block. In any case, drill a 1/8" guide hole so that when using the guide, the guide hole will be exactly in the center of the plywood edge.
Draw lines on the top edge of the chair sides, then using the drill-bit guide, drill 5 holes straight down into each side.
Make the arms from 1×4 fir or pine. If you plan to pad and cover the arms, this wood doesn’t need to be the best quality, but if you’ll leave it exposed, choose decent pieces of solid wood.
Now position an arm with the front of it overhanging the front of the chair side by ½". This overhang is necessary for stapling the fabric if you pad the arms. Mark the bottom surface of the arm to line up with the holes drilled in the top of the side. Drill the screw clearance holes from the bottom, which should ensure that the screw holes line up exactly. Repeat for the other arm.
Countersink the screw holes from the topside of the arms so the screws will be slightly sub-flush. Mount each arm with five #8×1½" flathead wood screws.
If you don’t want to pad the arms, you could round the top-front corners of the arms with a ¼"-radius router bit, or just sand something close. If you do want to pad the arms — highly recommended for comfort and appearance — that will be the very last step in building the chair.
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