Where to begin with the Rayleigh Dissipation Function?

I’m a final year student of mechanical engineering. Calculus of Variations isn’t a part of our syllabus.

I’m trying to understand the Rayleigh Dissipation function, and I understand that this is a part of Lagrangian Mechanics, but I’ve tried to understand the latter by reading Classical Mechanics by Goldstein, and Mechanics by Landau-Lifshitz, to no avail. The material on youtube either skims the surface or is incomprehensible. Any idea what the best way to learn Lagrangian Mechanics is for a complete beginner?