The cardinal rule for building a strong relationship with a stepchild is this; let the child set the pace.
So how do you do that? Gauge their level of openness to you and match it. If they jump into your lap or want to give you a hug don’t leave them hanging. But if they bristle when you try to hug them, back up a bit and find something less intimidating. If they call you daddy or mommy by all means let them. But if they would prefer to call you by your first name, that’s okay, too. Remember, letting the child set the pace is just how you start. It will grow well beyond that over time.