In my opinion, graded method books are the best source. Method books are available for many bass clef instruments, including trombone, bass, and cello. Graded method books are in multiple volumes, where volume one starts at an absolute beginner level, and after three or more levels will escort you through beginner, intermediate and advanced levels. The instrument specific teachings are irrelevant. All you really need is an explanation of standard notation and examplesthat proceed in an ordered fashion from beginner to advanced,
Graded method books are available where band and orchestral instruments are sold, and print music stores and websites.