There’s an excellent website for grammar and punctuation: GrammarBook. This subject of capitalization is complex and lengthy for the English language. As a tutor, I have always been confounded by how few words seemed to be capitalized in the Spanish language that the English language seems to give great reverence to. Hispanics don’t capitalize their own language, let alone days, months, peoples, races, and religions. I even have some difficulty getting my students to capitalize the first letter of the first word of a sentence.
There’s not a lot of theory here; just a lot of practice. I have a download below that spells out the rules of capitalization quite well; almost too well, and it’s a little scary. Honestly, it’s straight out of GrammarBook. There was no way I was going to tackle this Mount Everest of subjects. (We capitalize important mountains.)
Give it a read, and then introduce it to your student in phases. You throw the whole thing at them and I guarantee you none of it will stick. (Notice the evolving nature of website titles as in GrammarBook.)