Position your finished stitching over the wadding and card, glue the sides and pull the fabric over tightly then continue with the top and bottom before neatening the edges. If using PVA glue you may need to peg the sides to hold it together whilst it dries.
For the reverse panel follow the same steps as the front panel with only one layer of wadding. Top, bottom, sides and neaten the corners. Again peg to gold whilst drying when using PVA.
Apply glue to both halves and then sandwich together. If using PVA you will need to either peg together as before or apply a heavy weight (maybe a stack of books) just to make sure the two sides bond.