Hi,
Stain it first with the yellow and then I strongly urge either LMF walnut stain or alkanet root. Once stained, apply first coat of finish and sand the wet oil with 220 grit paper to build a slurry of finish and sawdust on the surface. Let that dry, and sand off with 320 paper. That should fill the grain but look carefully to see if you need to repeat the filling process. After filling the grain, apply the rest of your coats of finish.
This is black walnut with yellow and alkanet root but finished without filling the grain because it is a musket:
This is black walnut with just yellow and filled grain:
This is English walnut with alkanet root and filled grain:
dave
