The most direct route is state 21 west from Boise directly into Stanley. The western end of that drive is mountainous but most of the way is a nice two lane highway. LOTS of good camping along the Payette River and again, along the Salmon. Of course you said you were heading for Red Fish so probably planning on camping there.
You can miss MOST of the mountainous part of 21 by heading North out of Boise on state 55 then when you are a few miles north of Horse Bend turn east to Lowman. Pick up 21 again at Lowman.
Good luck / Skip