Very carefully. I haven’t done the bottom part yet as that was my first try but you can position the thread so it isolates only the hairs you want (see the part where she puts the thread under the ones she wants and pulls them to the side just to demonstrate) and slowly do a few hairs at a time. Letting go of the thread if it starts heading for areas you don’t want and starting again in the right place.

Watch videos and practice on your upper lip first.

Don’t be afraid to just finish off with tweezers in the tiny arched bits while you’re getting the hang of it.