Board and batten is a type of siding where the joint between boards appears to be covered by a smaller piece of wood. A board-and-batten-style door references this form both by a V-shaped groove at each board and by the mounting of the boards onto battens. In this case the batten is not attempting to cover the boards joint in a parallel way, instead it crosses the boards perpendicularly or at an angle.
Determine the width of your door. Your door boards are made of tongue and groove floor boards. Lay out your boards so that you end up with more than half a board on each end of your width. Each board at the end of your width should be the same size. This will determine how many boards you will need.
Cut your tongue and groove boards to the correct length. Run the face side of each board through your router twice and shave off a 1/4-inch 45-degree angle. When you join your tongue and groove together this will create a V-shaped groove on the face, creating the look of a board and batten face.
Cut three batten boards to length (the width of your door). Cut the top edge of each batten at 40 degrees (to shed water). The top board will mount 6-inches from the top of the door. The second board will mount at the centre line of the door. The third board will mount 6-inches from the bottom of the door.
Prime and paint your boards, including the tongue and grooves or stain and finish your door boards. Allow 24 hours for the finishing coat to dry.
Place your top batten on your work surface so that the narrowest width of the batten board is facing down and the bottom of the board is square. Position your outside tongue and groove board square with the side of the batten and 6-inches above its top edge. Drill two pilot holes through your tongue and groove into your batten. Nail two long nails through the batten. About 1/4-inch of nail should protrude through the batten.
Punch the end of the nail over and drive the tail into the wood. Do this for each nail. Mark the location of the centre batten and bottom batten and nail the tongue and groove board to all three battens. You will need to turn the door back and forth to peen the nails over. Align your next tongue and groove board and repeat. Make sure to keep your battens perfectly perpendicular and straight. Repeat for the whole door.
Turn your door face side down. Determine where your hinges will mount. Measure, mark and cut two diagonal battens. The high end of your top diagonal batten should be away from your top hinge location. Finish all sides of your two new battens. Fit each into position and then drill pilot holes through your tongue and groove and nail through, peening and punching the nail ends just like you did for your horizontal battens.
Do not add glue to your tongue and groove as this will cause the door to distort and come apart when the wood expands.