Apply a thin coating of wood glue to the corners of two adjacent sides (short and long sides) of the un-joined frame.  Place the two sides face down on a clean and smooth hard surface.  While holding the aligned corners together, place the plastic insert (thumbnail) into the groove. If necessary, you can lightly tap the insert down into the slot with a small hammer or mallet.  Check the face of the molding.  At this point you can slightly twist the corner if necessary in order to achieve a flush fit before the glue dries.

Repeat the above step on the other two adjacent sides. Allow these two "L" sections to dry for a brief period of time and then join the two "L" sections together.  Apply glue to the remaining exposed corners.  Holding the two completed "L" sections together face down, complete the frame assembly by inserting the plastic inserts into the remaining corner slots.   Check the corners to ensure a flush fit and allow the glue to dry according to the manufacturer's instructions before fitting your artwork into the completed frame.