First of all, check out the GitHub Pages guide
This is written with you having read that in mind, but just spelling out things for the slow people (me).
Having installed the prerequisites and cloned the repo, go to the local directory for the repo. Open the terminal there.
To run Jekyll, you need a Gemfile. Create it, such as through `touch Gemfile`, then add `gem 'jekyll'` to add that as a dependency.
The rest of the instructions are clear as is.
Buuuuut for ease of copy pasting >:3
bundle exec jekyll serve
To add new gems, remember to use 'gem install'
http://localhost:4000
Check out the Midnight theme's repo. There's some useful instructions there for locally using themes.