Your next step is grunt installation. Install grunt on your devbox.
To be able to use grunt you need to register you theme inside dev/tools/grunt/configs/themes.js. In many projects we add a version of the theme.js to the theme and then map it inside the composer.json of the theme:
If you use grunt this is probably currently the best way of working Increase grunt compile performance to 6 second
If you would like to work with gulp, please, see documentation – https://bitbucket.org/vaimo/module-frontend-workflow/overview
Please, install grunt or gulp to your project.