With the legs cut out and surfaces prepped, it was time to undertake the only real joinery of this project: cutting the mortises for the seat rails' tenons. These were nine millimeter wide, and would have to be located on the same location as the original chair's legs.
On site, I marked the height of the two tenons using a knife and a square.
I then marked the dimensions of the mortise from the original legs. I don't have a mortise gauge, so I just used my homemade cutting gauge, taking multiple passes after marking both legs.
With the mortises marked, I took the legs to my workshop and marked the cut more heavily with a knife.
I chiseled out a recess to guide my cutting, and then drilled pilot holes with an 8mm bit, hogging out as much waste as I could.
Clearly I don't have a 9mm mortise chisel, or any mortise chisel for that matter. I don't even have and 8mm or 9mm bench chisel, but the 1/4" chisel did admirably for the ends and the width, while the broad sides were pared down with a 3/4" chisel.
The front rail fits just fine!
I took the front subassembly upstairs to mate it to the rest of the chair. The side rails needed to be cleaned a bit of excess glue, and the mortises lightly shaped, but the final fit is nice and tight.
In situ. Next: shaping the legs, gluing it up, making a seat, finishing.