Assuming you have npm installed, the steps are:
When starting, it will trigger a chrome window where the tests will be run on the background, the only thing you have to see is the console. Personally I use sizeUp to change the size of the windows so it's possible to see the editor and the terminal at the same time. This way, there's no need to keep switching between windows and refreshing the browser all the time.
Karma will watch your files for modifications and run the tests on every file save, and because it's not running the whole browser, it'll take a ridiculously small amount of time to run all the tests. Instant feedback on every
Cmd + S. Awesome.