If you want to install gingerbread trim so the screws don?t show, drill clearance holes through an open part of the trim towards the rake boards or fascia boards of the house. Drill pilot holes into the fascia so that you will not split it with long screws. Then screw it into place.
If you need more support you can screw a backer board behind the fascia and trim. In fact using a backer board or metal straps can allow you to avoid trying to screw the gingerbread edge to edge with the soffit boards.
Use spacers if necessary to keep the front of the gingerbread and the front of the raker board flush.