If you did everything as done in the video.
Simplest way to carbonate is to just put the cap on the bottle once the bubbling has slowed and the balloon shrinks. When the plastic bottle gets hard or starts to swell release some pressure. You can the refrigerate and serve from the bottle, taking care when serving not to disturb the yeast on the bottom. I wouldn't recommend this way unless you can check on it every few hours.
Or you can let fermentation complete and let yeast settle, then transfer to a clean bottle, add 3/4 oz corn sugar and seal bottle, leave to ferment again making carbonation (7-14days). Chill and serve