(1) Find two points along the wall from the ceiling, measure 50 cm each, and make two marks.

(2) Connect two markers and draw a thin line.

(3) Then find out two points A and B according to the size of the photo frame, so that A and B are the places where nails are nailed.

Reason: AB is parallel to the ceiling line, because the ceiling is parallel to the ground, so the photo frame is horizontal.