I employed various chisels and files, and worked in from the top and bottom of the plane in order to open the mouth. It can take awhile because you need to be sure that all surfaces are as flat as possible.
I found it a little too iffy to cut the final angle of the abutments by following the pencil lines that I had marked on the outside of the plane. So I cut a 12º wedge from a thin board to use as a pattern to mark the angle on the inside of the plane.
After the abutments were cut to shape, I rough-cut a full size wedge on the band saw and cleaned it up with a plane. The fit of the wedge needs to be spot on in order to hold the blade in place, so there is a bit of checking, tweaking, and rechecking involved.
Once the wedge is properly fitted, some of the lower portion needs to be removed in order to provide a clear exit for shavings.
At this point you have a working, but extremely painful to use, high angle smoother.