After deciding exactly how you want your pieces to intersect carefully mark the location of the intersection points for later reference.
Hold or clamp the pieces down firmly (I had to use my other hand to take the picture, but for real held the top piece in place with my other hand). Score a line along the curved edge where the pieces intersect with a utility knife. Don’t try to cut very deeply or you will likely slip and mar the piece.
Repeat this process on the other side. I didn’t take a photo of this but you can use the same process.
Use a square to mark a square line down the side so you will be able to keep the sides of your shiplap straight.
I then went to my bandsaw and made a series of multiple cuts to half the thickness of the piece. This makes it easier to chisel out the majority of the dado before worrying about the curved edges. You could do this with a small backsaw if you don’t have a bandsaw.
In the next post we will begin removing all the material with some careful work with a sharp chisel.
-- Kevin, Blue Springs, MO