5) Once I cut the tenon, I fit it and then confirmed the width of the housing, so that the entire base of the splat fit 3mm into the shoe. The housing needs to be snug. That makes the plants on the splat look like they are growing out of the chair rail, which is what we want.
6) Once satisfied with the fit, the first of the four sides of the chair back was done. The other three sides needed to be addressed, one at a time. Either the left or right leg needs to be fitted next. Do not try doing the crest rail second – there is no reference surface to get the alignment correct.
I chose the right leg. Fit the splat into the shoe and clamp it. Install the chair rail in the leg. It will need to sit a bit proud, since the splat will hit the leg and hold the rail away, <photo 7>. Notice the gap between the chair rail and the leg. That must be eliminated. Mark the contact location. This is where the mortise needs to be cut. My splat is ~22 mm thick since much of it will be carved away. The connection to the leg needs to be strong, but not 22 mm thick. I cut some thickness off of the front or back of the splat at each connection to get a 15 mm thick tenon. Also, consider how the splat will fit into the chair leg. Cut the splat shape so that it will slide squarely into the leg, and also respect the grain of the splat. The picture of the splat in photo 7 does not reflect those comments. The splat tenons into the crest rail in <photo 9> have been squared for better insertion and strength.
7) I drilled out the mortise then used chisels to cut the edges to shape. Cutting the mortise deep is OK. Test fit and adjust as necessary until the chair rail is flush against the leg, <photo 8>. Any increase in the gap between the chair rail and the chair leg than was there with no splat in indicates that the splat is holding the rail off the leg. This was fiddly, but fitting the first one on each chair was the hardest.