From your photos, looks like the hinge bracket that's attached to the car has 4 small holes in it. Guessing you have to either squeeze and/or lever the two metal springs on the back of the flap so the ends of springs locate into the 4 holes. The springs, or one of them, may go in a certain way under tension to hold the flap shut - but it's hard to tell from here. May be a bit fiddly, but doesn't look too bad.