You've found the perfect man or woman for you, but nowadays, many people in a relationship have children from a "previous". Here's how to be a step-parent to all ages with as little conflict as possible.
Don't expect them to call you Mom/Dad straight away, if at all. After divorces or the death of a parent, most children don't understand the logic behind their Mom/Dad marrying again.
If you don't approve of your husband/wife's previous spouse, don't say so in front of the children. This will just cause them to either dislike you greatly or parrot to the parent in question, causing unneeded trouble.
Don't be upset or disheartened by negative reaction. The child hasn't got a disliking of you as a person. Their childish logic sees you as an enemy for trying to be a new Mom/Dad (although you are not), and they may take their unhappiness out on you.
On the other hand, if the child is upset and unsettled, and wants to talk to you, embrace it. If they see you as approachable and sympathetic, they will ultimately be less hostile.
In the case of teenagers, they may say "You're not my Mom/Dad!" when confronted with rules. This is common, so don't snap and shout back, simply be consistent and don't waiver, as teenagers will see it as a sign that they can walk all over you.
Be happy! Children prefer a happy and approachable adult to speak to.
Meet your boyfriend/girlfriend's children first, don't just turn up after dating their parent for a long time.
Find some interest that you have in common with the children. For example, if your partner's teenage daughter likes music and you play guitar, offer to teach her, or if she likes books, talk about some classic titles with her.
The children may never fully accept you. Be prepared for that.
It will be tiring and upsetting at first, but persevere.
Shouting back in an argument with the child will just make it worse.
Never criticise the children to your partner, they will see them as faultless, as they are his/her children, and he/she has known them all their lives.
Never order the children to call you Mom/Dad. This will just make them more determined not to.