When attempting to fit the flap, it can sometimes be made a little difficult because of excess paint at the hinge. Carefully remove this using a small, sharp file like the one in the picture and keep trying the hing fit. When it slots in, apply a little grease and secure it in place with the correct brass tube hinge pin.
You will see we have covered the area around the hinge with lots of masking tape (just in case). Push the pin in to the frame hinge loop and tap it home with either a pointed driver or small centre punch. Once it is in position, it needs to be pushed against at one end whilst the other is slightly flared. Do this to both ends. This prevents the pin from working loose and falling out.
You may need to manipulate the flap a little to get it completely square with the frame and ensure it opens and closes properly.