Using a mallet, strike the tool directed along the edge of the bent portion.
Start with small hits until you develop a sense of how much force is necessary.
The idea is to just stretch the metal back into position, a little at
a time using the curved base tool as a mandrel.
Severely bent rings will have too much metal deformation for a perfect repair
but you should at least be able to get back to where a filter can be installed.
You may need to bend back a little oversize to account for thread distortion.