Get the highest resolution scan that you can manage. Break it into glyphs. Select your representative glyphs.

Become frustrated that in eighteen pages, there are no examples of [#%|\=÷×{}!*/] or a capital X or Z or five digits. Find substitutes.

Use FontForge. Read the whole FAQ.

I’ve used Glyphs before for drawing type and can’t recommend it enough. Reasonably priced, quite powerful, fast and fluid to use. Drawing—even re-drawing—type is almost addicting. When you get the Bézier curves dialed in just right it’s so satisfying!

glyphtracer [1] can help with this, but letter shapes are only a small part of what makes a typeface. (Typography is about whitespace, or as Miles Davis said, “it‘s not about the notes you play, it’s the notes you don’t play that matter”.)

[1]: https://github.com/jpakkane/glyphtracer