The mortises in the violin corner blocks, which will secure the linings in place, only need to be 7-8mm deep and maybe 6-7 long. A little marking tool like the one below is helpful when marking a pencil line to that 7-8mm depth. I guess you should settle on an exact number before you make your marking stick. 7.5 perhaps?
Note that on the top and bottom blocks you just cut a shallow corner, then cut the ends of your linings to fit with that angle.
A really sharp, thin, and strong exacto knife works really well to cut out that mortise.
Juan just uses a chisel he made out of nail to pry that little chunk of wood out.
And then you can test the fit of the mortise in the corner block with a scrap piece of your lining.
Next Step: Bending and gluing in the linings.