My "sperm donor" abandoned my mother and I when I was 2, so I have no memories of him whatsoever. That being said, he never paid CS either. So I have to agree with the above, loving them and being there for them is the first step. My mother met my stepfather when I was 7 or so, so I guess you could say he's been in my life for the last 21 years. From my perspective I wasn't sure how to handle him. I mean, here I am just my mother and I, and we're getting along just fine. She'd had a few boyfriends when I was younger, and only one that stands out. Partly because when they broke up he just vanished. Don't recall why, but I remember liking him. Because he reached out to me. Really, all you've gotta do is be there for them the way I see it.
To spare the cliché of "don't replace their father," I'll just say treat them as you would your own children. My stepfather and I initially bonded over my love of video games. Over the years we've had our differences, but I honestly can say that man is more of a dad to me than I ever expected. We don't always see eye to eye, but I've learned to respect him. He was always there with advice, when I needed a friend and nobody was there, and wasn't too afraid to be the parent.
But again, I have to agree with the above. You sound like you're on the right path. I'll leave delving into my own personal history for another time