Connect the sides (1200 mm x 50 mm) x 2 to the top (1200 mm x 264 mm). Start by drilling pilot holes and countersink them before applying wood glue and fastening the pieces together with screws.
Connect the legs (264 mm x 432 mm) x 2 to the underside of the top, between the sides. Measure 200 mm in from each end of the top and secure the legs in position with screws and glue.
To give the legs extra support, secure the 4 triangles (200 mm x 200 mm, cut at 45°). Drill pilot holes and countersink them before applying wood glue and securing the joints with screws from the sides as well as from the outsides of the legs.
You are now ready to fill the screw holes with wood filler. Leave the filler to dry before sanding the unit to achieve a smooth finish. You can now paint the unit with a coating suitable for outdoor use.