Check the fit of everything and confirm that all the pieces mate snugly. Before assembly, you could perform some sanding at this stage. There are some areas that will be difficult to sand after assembly so you can sand at this stage, but you need to be careful not to excessively sand the joints. You should also refrain from creating round-overs on those edges that will mate with another frame piece.

Next you are ready for assembly. Confirm and mark the orientation of the pieces so that you don't have to worry about this when the glue is out and drying fast. We used polyurethane glue and applied a small amount on the tenons and clamped the frame together with bar clamps. Confirm that the frame is square and let it dry. A good way to check for squareness is to measure the diagonals of the frame: if the dimensions are identical, they the frame is square.