Cut front rails. Cut the top and bottom front rails to size from a 1x6 board. For our standard 36" door replacement, the needed width was 35".
Cut back side stiles. Cut 2 back stiles to length from a 1x4 board. This was 35" for the bottom panel and about 43" for the top panel - adjust those measurements to fit your door height and desired split height.
Attach back side stiles. Attach these 2 back side stiles to the front rails using one screw only in each. Square up the frame using a speed square and/or a rafter square.
Square up the frame. Check the frame for square with an easy measuring tape trick. Measure one diagonal, then compare it to the measurement of the other diagonal. If the piece is square, they'll be the same length.
Adjust the pieces (by squeezing or pulling apart the corners) until the measurements match, then secure the back side stiles with additional screws to keep the frame square. Remember to line up the screws neatly.
Cut front stiles. Measure and cut the 2 front stiles to length from the remaining 1x6 board. This length will be the height of the half door minus 11" (5 1/2" each for the top and bottom rails).
Attach front stiles. Attach the front side stiles through the 2 back side stiles using screws. Remember to line up the screws neatly.
Cut X crossbrace. Place the 1x6 crossbrace board under the frame at an angle, matching up board edge with the corners. Overlap the ends slightly to provide part of the board to be cut off. Use a pencil to mark both inside edges where the frame crosses over the cross brace. This gives you the exact angle to cut your board to fit the frame. (Be sure to mark the piece on the wood that will be discarded.)
Cut shorter cross braces. Repeat the process for the second crossbrace section, marking and cutting the 2 smaller crossbrace pieces. Leave the crossbrace pieces in the frame - they'll be attached through the back stiles in the next step.
Cut and attach back stiles. Cut the remaining 8 back stiles from the 1x4 boards. Lay them over the top of the crossbrace pieces. Attach them to the front rails and crossbraces using screws - and remember to line up the screws neatly.
Add top cap / shelf ledge (lower door only). Rip the remaining 1x4 piece to 2 1/4" width on the table saw (unless you are using a 1x3), and cut the cap to length. Screw or nail on the cap. I used brad nails and glue so they wouldn’t show.
Repeat for the second half door. Repeat the building process (minus the top cap / shelf) for the upper half of the door. Measure carefully to build the door to your needed specifications, leaving 1/2" allowance for the doors to swing freely.
Sand and stain the 2 half doors. We used Minwax water-based Charcoal Gray, then colorwashed the doors with Sherwin-Williams Iron Ore paint to match our painted interior doors.
Hang the doors and install Dutch door hardware. It’s typically easiest to attach the doors together and hang them (with the extra hinge) like one door. Install and hang the hinges, latch(es), and handle(s).