We start at the point where we have a 18 mm wide welt, OK?
Mark 4-5 mm wide with your thumbnail.
Then skive! Make sure you leave the half of the thickness and the angle of the knife is as steep as it is possible. That creates a nice profile because of the edge curvature.
Remove the grain side a bit with a piece of glass.
You can press a small channel to the grain side to help the stitching go in the same line.
Then skive it back from the flesh side – but make sure you leave 1 mm thickness!