You asked about the application - well here is a great tip that I have learnt from trial and error:
- apply the glue as if there were no flaps, just straight across the hairline
- apply the hairsystem
- now check the covered area for the flaps and apply glue to that exact area
- neatly press the flaps down into that area

This way is hassle free. It's very difficult to apply the entire piece + flaps to your head all at once and it can be messy, often requiring you to reglue the flaps if they ended up landing in the wrong place.