While trying to study textbooks on analytical mechanics or QFT I realized that I simply cannot operate with variations of functions in the same way I can operate with derivatives and integrals. I have never learnt calculus of variations in university and, frankly, I am not much interested in this topic by itself, but I need it to go further in my attempts to learn physics. So can anybody recommend a good practical book or may be video-course on this subject? Again, I do not need very fine details and rigorous proofs, but I need to gain practical ability to understand this stuff and gain some intuition with it. I see that some physics textbook try to introduce calculus of variations just as part of the text, but I fail to understand it to a satisfactory level.