How do I become a good step-mum?

In the next couple of years, my plan is to be a step-mum. My boyfriend is 27 (I'm 21) and has two young children from a previous marriage. I really do care about these kids, and my partner and I have talked about marriage, so I've come to terms with the fact that in my early twenties, I could play a big role in raising them.

It's not that I know nothing about kids. I run a girl-guiding unit in my area every week, so I have at least a BASIC grasp of how to talk to a child. This is why I need advice from all the Redditors who either are step-parents or have step-parents, to give me pointers on what I should and shouldn't do. The last thing I want is their mother to be annoyed at how I'm handling them and make things difficult for my partner and I, I really don't wanna mess up here! So I'm asking you, Reddit:

How do I become a good step-parent?