Sometimes I just build my own hairline by carefully plucking individual hairs after I`ve applied the hairpiece.

Get the hairs good and wet all the way soaked to the base, then use tweezers or something with a good grip to get a hold on the hairs and pull them slowly until the knot comes untied and keep pulling slowly until the hair is pulled out.
Do the same with some of the other hairs in a staggered and random fashion to recreate a natural hairline.

You want to pull the hairs out by the knot. You dont want to pull too fast and break the hair, ir you`ll be left with the knot and a short hair, get a bunch of these and it`ll look out of place.
It may be necessary to to apply some conditioner so the knots will loosen up.