Step 1.

Lay the five 200x25 x2075mm (1x8 x83") vertical door slats next to each other on a flat surface forming a rectangular panel. See fig.1.
The width of the rectangular panel will be wider than the required door width at this stage.

Measure and mark the required door width of 900mm (36") central on the panel and cut off (rip down) the excess each side.

Step 2.
Glue and screw two 200x25(1x8) horizontal boards (rails) to the vertical door slats, one at the top and one at the bottom. See fig.2.

The length of the screws should be slightly less than the combined thickness of the rails and slats.

Screw diagonally with two screws at each meeting as shown in fig.2.

Step 3.

Cut to length and glue and screw two 75x25 (1x3) battens to the slats at each side of the door ensuring that the edges are flush. Screw in a zig-zag pattern as shown in fig.3.

Step 4.

Cut to length and glue and screw the middle horizontal rail in place between the two side vertical battens. The height placement is just personal preference although middle rails are usually placed within the top half of the door. See fig.4.

Step 5.

It is now just a matter of cutting to length, gluing and screwing the intermediate 75x25 (1x3) battens to the door slats covering all of the joins. See fig.5.

For instructions on how to install the door go here.