Well, I'm sure there are fancy videos and all that, but I'd just grab a standard text, like Walter Piston's Harmony and do it.
I just picked Walter Piston because that's what I'm familiar with if I remember way back, and at one point it was the dominant textbook for undergraduate-level common-practice-period theory.
I personally began studying music theory at about age twelve or so, as part of a school curriculum, taught one-on-one with the school's MusicDirector and factotum, and it involved ear-training using some primitive (by today's standards) computer software, and writing endless counterpoint.
So, that's all in a standard textbook like Piston's, but I would supplement that text by nabbing some PDF syllabuses from UG courses so you might have some kind of an order to proceed.
But, books like those are ordered generally so you can start with Chapt. 1 and just proceed.
Do the exercises, though, with pencil and paper. I don't recall what kinds of exercises Piston's book has, if any, but I'm sure they are there.
And, you can always have somebody here "grade your homework" to see if it's right.