If you don't have Xcode installed (for OS X), install from the Mac App store. Once installed, add command line tools by going to Preferences > Downloads > Components.

Next, you will need Ruby. You may consider using a Ruby version manager such as rbenv or rvm to help ensure that Ruby version upgrades don't mean all your gems will need to be rebuilt.

On OS X, you can also use Homebrew to install Ruby in /usr/local/bin, which may require you to update your $PATH environment variable. Once you have brew installed, here are the commands to follow to install via homebrew in terminal:

$ brew update
$ brew install ruby

• Once you have ruby installed (see above)
• Open terminal
• 'cd' to directory (leave a space after 'cd', then drag and drop your site folder into the terminal window)
• Install Jekyll by running:

gem install jekyll

• Run site by using:

jekyll serve

• View the site in your browser by going to http://localhost:4000

Any changes made locally will cause the site to rebuild automagically.