Inorder to 'skin' the flats (basically covering the front in a piece of ply wood) we covered the frame frames in PVA wood glue and then laid the ply wood skin on top of this. We then used an electric nail gun to fasten the ply wood skin. To avoid the ply wood and frame not being flush we worked from the 'floor' edge of the flat and then up one long side and then the other long side and finally the top making sure to correct any inaccuracies that may have occurred during the the making of the frames e.g them being wonky. Once the outside of the skin had been nailed on, the inside frame supports were nailed to the skin.
(skin being nail gunned to the frame)