Cut the mark area.

 

Smear the back side of front wall with glue twice

Smear the edges of facial side of front wall with glue

Glue the details to each other

 Press the parts to each other

Mark the line of cutting

Scratch along the marked line by awl several times

Make the cut by the scissors under 45º angle

Break along the scratched line

Sand the line for gluing the corner of the building under  45º angle

Smear the surface with glue twice