To finish the assembly of the miniature table top, use a glue spreader to spread a thin layer of glue along the narrow upper edge of the apron rectangle, after the rectangle is thoroughly dry and glued solidly together.
Line the glued edge of the rectangle up with the rectangle you drew on the underside of your table, so that the rectangular apron is an even distance away from the table edge on all sides. Press the apron down onto the underside of the table and use a glue spreader or strip of waste plastic or card to remove any excess glue from the join. If you have a plastic cutting board, lay that on top of the apron and weight it down with a book to help clamp the apron to the underside of the table top.
Leave to dry.