Stand on the side of the armoire that you want to install the bookcases upon. Measure the width of this side and the height. Purchase bookcases that are no longer than 3 feet high and no wider than the width of the armoire. The bookcases will go all the way to the ground, but when on top of each other, generally should not exceed the overall height of your armoire.
Turn one bookcase over to expose the bottom. Drill one end of L-brackets to the far right bottom side and the far left bottom side with a handheld drill and screws. Use round headed screws that are 1 inch long.
Turn the other bookcase so the top panel is upright. Drill one end of L-brackets to the far right top side and the far left top side with a handheld drill and screws.
Place the last bookcase you drilled L-brackets into against the armoire, touching the floor. Drill the other end of the L-brackets directly into the armoire.
Prop the other bookcase directly above the bottom one and drill the other end of the L-brackets directly into the armoire.
Drill the back panel of this top bookcase directly into the armoire by drilling screws into the top right and left corners.