The rub mix will make ⅔ of a cup.
Scrub the ribs under cold water. Damp dry with paper towel. Sprinkle the rub on both sides of the ribs. Cook on low for 6-8 hours or until the tenderness you desire. If you want BBQ sauce on them add 1 hour before they are almost done. All crock pots are different as far as heating temperature. Mine is very hot on low and it only takes 6 hours.
Notes: Does not require water! And suggestion from comments to stand ribs vertically on perimeter of crock pot.