I posted a pictorial on it a few years ago.
Here is a condensed version:
After the flange has been straightened the post can then be easily inserted into the new plastics piece.
Then install the plastic to the playfield and as you tighten the screws they will mushroom the post again to the plastic. If the piece is a lower plastic (like this EBD lane guide is), you can use a shorter screw to mushroom the post and then remove it or leave the post as-is because there is another one that goes on top and after the screw is tightened it won't be going anywhere.
If you are afraid of stripping the holes in your playfield trying to mushroom the post you can use a separate piece of wood or make a metal fixture (a piece of metal with a tapped hole) and use a machine screw to do it and then install the plastic on the playfield.
Of course if you want to get real fancy you can use a rivet tool to mushroom the post too.