I’ve been a step-parent for 15 years (and DH a step parent to my girls the same length). My situation is a bit different to a lot because DS’s Mum died when he was a toddler so there’s only DH, and then me.
Have honest, brutally honest, conversations with your partner before living together. It’s not the big things that cause the resentment and long term hassles, they can often be sorted, it’s the little day-to-day things that niggle away at you.
Be realistic about how tough it will likely be at times (and I actually think people need to be a bit more realistic sometimes about wanting to be a step-parent - if you really, really, really don’t then walk away just as you would if your partner had another issue that you wouldn’t want to deal with).
If there is any chance at all you will have kids then discuss your non-negotiables and get them in place from day one. A lot of step-parents I know hit major issues because they wanted lots and lots of things changed once they had their own child and that’s the worst time ever for making changes.
For dealing with the actual step-child always be consistent. Don’t be their best-mate/favourite Aunty type one day and then harsh step-mother the next. It’s confusing
Don’t expect too much too soon. Kids need time
Give the child time alone with your partner
Equally, especially if they live with you or do 50/50, once you get to know them consider having something that you and the child do. My relationship with DS was partly built on our Sunday morning walk to the local shop to pick up breakfast rolls just the two of us.
Try and remember that they’re not being difficult because they’re step-kids. They’re being difficult because they’re kids.
I think the main one is “be realistic”. We have a great blended set-up, but it took a lot of hard work and there were bumps along the way. It’s not always easy and if it’s not for you then walk away.