First thing's first: if you're watching on an Apple TV or some smart TVs, there's a chance the interactivity function won't work. But it does work on iOS and other devices (so, worst case, I guess you can watch it on your phone, aka the original "black mirror"). As you venture through the story with Stefan, two options will occasionally pop up on the screen for you to choose between. Once you've decided, the film will smoothly continue, without pausing or fading to black (which is actually pretty cool, and helps make Bandersnatch feel as much like a true film — rather than a game — as possible).
If you find yourself struggling to make a decision — you only get so long, so you can run out of time — Bandersnatch will end up choosing for you at random. But be warned: once you make a choice, that's it; you can't rewind to pick the other option. However, you'll discover that a few do-overs here and there are available, and there's always the choice to start the entire film over again to wipe your slate clean.