I have a dry, powdered pigment. It does not list a fixer or the ratio to mix it at. I think IPA should work, but am left with the ratio problem. Can anybody help? Thanks.

There is no real uniform ratio. Mix it in small bits to get the consistency you want. If you want to paint it on (so it creates a uniform caking of dust/grit) then the consistency you want should be as thick as (roughly) Tamiya paint. Less fixer/thinner means a coarser finish, more fixer/thinner means a smoother, finer finish.

You can also mix pigments with water and then, once you are happy with the coverage, apply the fixer.