Cut panels of the appropriate dimensions for the two slide-out shelving units.  Install a dado blade on the table saw and cut two dados/grooves on the interior surfaces -- one on the left and one on the right. Pre-drill them for pocket-hole joints at the corners.

Set shelf-standards (manufactured metal tracks for the movable pins that will hold up the small adjustable shelves) into the grooves, tapping them gently with a mallet to seat them in the groove. Screw them in place.

Apply glue to set for the first unit, then clamp the sides, top, and bottom together and secure them with pocket screws.

Set the fixed center shelf in place. Use joining clamps to position it, and then secure it with screws.

Repeat this step to assemble a second shelving unit.