I guess my main concern is that I do this how God wants me to. I have been praying for guidance and reading a lot about Plan A. Thanks for all your advice!
Exposure is your greatest weapon against adultery. Affairs thrive on secrecy so exposure is like chemotherapy to cancer. God would want you to defend your family from this assault and stand up for them.
Have nothing to do with the fruitless deeds of darkness,
but rather expose them. Ephesians 5:11
I would simply get on the phone and start calling his family members. Tell them the facts of the affair, tell them you want to save your marriage, and [very important!]
ask for their advice. When you ask for their advice, they have a tendency to want to help you.
Your children should also be told. Children can deal with the truth, they cannot deal with lies. Your H can explain to them LATER why he is committing adultery and hurting his family like this. But YOU should be the one to tell them the truth.
The OW's family - all of them - should be told that she is messing with a married man with children. If your mother could call her mother and ask her to persuade her DD to leave your H alone, that would be great.
If this is a workplace affair, then send a certified letter to Human Resources.
Everyone should know about his adultery. Exposure ruins affairs. It might not kill them instantly, but it sure hastens their death when everyone is looking at you with horror.
These exposures need to be done in the same day. That way you are not pre-empted by the infidels who will spin you as a "jealous nut who is imagining things." The story needs to come FROM YOU.