Make sure node and npm are installed. The easiest way is to install node via the available installers: https://nodejs.org/en/download/
Please ensure to use the 4.x LTS version instead of the latest 5.x version. For installing node and npm on debian based distros, these instructions might also work: https://www.digitalocean.com/community/tutorials/how-to-install-node-js-on-an-ubuntu-14-04-server
The easiest way is to use nvm. Simply install it as described here: https://github.com/creationix/nvm/blob/master/README.markdown
Afterwards, you can install node 4.4 by entering
nvm install v4.4.1
This will work very well for Mac OS X and Linux based systems. For Windows I recommend using the afforementioned installer.
Browse to the phpBB/ folder and enter:
npm install
NPM will then install the necessary dependencies. This might take a while.
Additionally, you might have to install gulp globally, too. For this, simply run:
npm install gulp -g
You can either manually trigger the compiliation using
gulp adm_compile_sass
or make Gulp monitor the style files and compile them if necessary using
gulp watch_adm