Turn the side table upside-down and use the drill and wood screws to attach one locking caster to each leg. Turn the cart back on its feet and lock the casters.
Set the butcher block centered on the tabletop and use a pencil to mark on the underside the placement for the L brackets. The brackets will be attached horizontally to the butcher block and vertically to the outside edge of the tabletop at each corner.
Screw the brackets to the table with wood screws and a drill, then set the butcher block back on the tabletop and attach it to the horizontal branch of each bracket.