There should be a steel support plate that is attached to the door (shown in the diagram provided by spartacus'). What holds the flare to that plate are 2-3 plastic pressure clips and 2 rivets. I had reaffixed mine recently (passenger rear door). I was able to locate two replacement clips from the salvage yards and secured it with two appropriate sized rivets for that hole diameter. If I knew the size, I would tell you, but I have an assortment of rivets in a box that are not labeled.
It isn't a difficult job. I would begin by removing the flare and determining if you have that metal plate behind it.