This is a great question! We follow up with Catholics the same way we follow up with everyone else.
What I have found in my experience with Catholics who come to know Jesus is that they know a lot about God, but just don't know Him. (I know I'm generalizing and that this can be said of many people, so please forgive me for painting with a broad brush.)
But one thing that I've seen in Catholics when they invite Christ into their lives is a hunger for God's Word. They want to learn God's Word. That's why any follow up plan for new believer MUST include a means to educate them in the Bible.
So here's what I suggest: Develop a plan for following up with new believers that includes a new believer's class as part of the process. This will increase you retention of new believers and will kick start their discipleship.
To learn more about the New Believer Follow up System we've developed at Calvary Fellowship, click here.
To pick up a copy of the New Believer's class that we use at Calvary Fellowship, click here.