Predrill for 2 screws in 1 end of a side. Apply polyurethane adhesive sparingly to end of adjacent end piece. Butt 2 pieces together, check they are correctly aligned, then screw together. Keep adding boards to form a rectangle, each time screwing a new board to end of previous board.
T&G boards have a tongue at top and a groove on underside, which allows boards to lock together. For the top edges of the upper boards to be smooth, plane off the tongues. In addition, as 2 layers of boards are staggered at corners, use a chisel to remove 20mm of tongue at each corner so next board can sit down properly on layer below.
Screw top layer of boards together as in the bottom rectangle. Use a wooden mallet to tap timber home if fit is a little tight. Alternatively, a soft piece of pine and a hammer will do the same job. Glue and screw a 19 x 19mm cleat into each corner to hold lower and upper layers together securely. Cleats should star t 30mm up from bottom of box and finish 5mm from top of box.
To form a base, from underside, screw together 4 boards using two 40 x 19mm base cleats. With box upside down, insert base so it settles on bottom of corner cleats. Use wooden mallet to ensure base is sitting down well, then screw in place through sides and ends.
Screw a 50mm castor to bottom of planter in each corner, if you like. This will allow you to move planter around to chase sun in winter, and shade in summer.
With planter still upside down, drill a pair of 13mm diameter holes in each end, about 40mm down from top and 180mm apart for rope handles. While you are at it, also drill 8 drainage holes in bottom of planter.
Feed sisal rope through handle holes from outside and tie a knot on inside so rope cannot pull through.
To protect planter and bring out rich history of colours, paint with an exterior oil or clear finish.