My car door just flops around!

The rear door on my 2001 Malibu no longer has the detents or stops that keep the door in one position when it is opened. It justs swings loosely. There is no problem opening, closing or locking the door; just the lack of the tension and the stops.

Can anyone tell me how to fix this myself?

The door check is worn out. This used to be detents incorporated into the hinge but most vehicles today use a bolt in check that connects the door to the post.