The best resource is here: https://live.rbg.tum.de/course/2019/W/semantik

This is a video lecture series by the author of Isabelle. He assumes some familiarity with functional programming like Haskel.

Also, if you get stuck somewhere, you should simply paste your question in this Zulip chat: https://isabelle.zulipchat.com/ . They respond quick (almost real time) so that you carry-on your proofs happily