Proofs

Typically, you will learn proofs if you study discrete mathematics. Khan Academy does not dive into great depth regarding discrete mathematics. There are free opensource textbooks on discrete mathematics.

Perhaps you could do a bit of searching. Alternatively you could look into purchasing a discrete mathematics textbook. 

I'm aware that exists. But some people like me don't learn easily from textbooks and Google searches. Khan Academy has the videos and interactive resources that make it so much easier to learn things without stressing you out.

(Simple) math came easily to me when I was in high school, it was an easy A, but one of the things I never "got" was math proofs, how they work, how to do them, etc. So, I would take a class like this.

Now, I have a closer look at the original post, I notice things. The exact structure of proof does not matter, it is up to you to determine this. Whether you want to do one long paragraph or two column proof.

There are lot of common techniques for proofs e.g. (induction, proof by contraposition, counter example etc) 

Khan Academy takes a very, very long time to create courses. In short, the best solution would be to use an open-source textbook or consider buying one from Amazon.

The book listed below is interactive/free.

https://discrete.openmathbooks.org/dmoi4/ch_logic.html