Former Christians that are now atheists?
Simple. Just put in the keywords " How To Evangelize" into Google. And follow what the “experts” are saying - expectantly from page 1 (as they have the most page ranking). 
Yeah the problem is that’s too ambiguous. I’d like to have resources on how to personally evangelize those who were once in the faith. This approach is going to be different than if it was a standard atheist that has never believed IMHO.
Try the keywords “how to personally evangelize those who were once in the faith” in Google. Three interesting resources appear on page 1:
en.wikipedia.org/wiki/Approaches_to_evangelism
equip.org/article/witnessing-to-those-who-have-fallen-from-faith/
evangelismcoach.org/personal-evangelism-101/
There is another tip for US residents. Call your local public librarian. Ask for adult reference. Then have them help you search for resources. But you can find, what you need in Google. If you use the right keywords.
And if you attend a church. Talk to the clergy. They will be happy to help.
I’m on a roll tonight…
If you are going to evangelize, then what are you evangelizing about? 
This is a evangelical universalist forum where many think that in the end, God will somehow bring all of humanity into His glory. So you may want to simply tell those you meet about the good news of what God did through Christ. He was the reconciliation between God and man.
Basic and simple.