I have been working a conlang for a few years now, and I initially found some free lists of words here and there, bought a book or two, but never really found them that deep.
Recently I encountered Lojban. If I were to start this process over learning from scratch, I would study Lojban. They tell you in there the history of how it was made, the problems they tried to solve, etc.. It's a pretty good glimpse into how to create a conlang. Here is a Lojban timeline.
I created a repo with some word/sentence lists to be able to know some things to cover at the basics. Wikipedia has a bunch of basic word lists, and there are some other spreadsheets floating around on the web somewhere.
But other than that, just wrapping your mind around the pieces of creating a conlang, there are several good books on this which I'm sure you've seen recommended (like the Language Construction Kit you linked to). Creating a sound system, maybe a writing system, creating a lexicon, and then creating the grammar (the real hard part, which there's not much out there for learning materials).
Toki Pona would also be another good conlang to study, for a minimalist perspective. Here is a word and grammer cheat sheet.