You need to install the hexo-cli globally and also install all the required node modules:
npm install hexo-cli -g
npm i --no-optional
That's it.
Write articles and save to source/_posts. Filename should be YYYY-DD-MM-your-title.md as this'll help keep things organized.
Stuff you're working on but are unfinished with can go in source/_drafts. If the repo is public, they'll be public, so, you know, watch out.
All of the "behind the scenes" stuff that you'll be working on is in the blog branch. The master branch contains the static site generated by Hexo. Be sure to git checkout blog if you want to do any work.
You can preview your changes while working on them with:
hexo serve
Write mind-blowing blog post and commit it to the repo normally. For example:
git add source/_posts
git commit
Then clean and deploy:
hexo clean
hexo deploy
The clean step isn't technically required, but I always have trouble with weird stuff not updating unless I clean first.