Build a healthy relationship with the biological parent. This is the most effective way to develop a good relationship with your step kids faster. This will take time to develop though; and you must be patient about it. Allow your stepkids to be with their biological parents but don't disappear in the scene. If they get into trouble, let your partner (their biological mother or father) to reprimand them.
Don't compete against the biological parent. Respect your step kids' connection with their biological parent. Also, respect how the latter discipline their kids. Don't fall into the temptation of trying hard to become a better parent for them. Avoid controlling your step kids or fixing their lives; rather, be present in their lives and provide the support they need.
Pay attention to the interests of your step children. What do they like? Ask questions and observe them. Find similar things that you love and do them together. This will create an impression that you're a reliable friend to them. Hence, it will be much easier for you to win their hearts and to become accepted as a parent by them.
Allow your partner and your step kids to go out together. Your step kids might feel displaced because of the new setup they're in. Let them enjoy some peace and quiet with your partner. This will help them sort things out and reestablish their relationship with each other.
Treat your step kids with care and respect. It is difficult to truly love your step children. But you don't have to show or make them feel that you hate them. Your step kids will not truly love you either. So, be composed enough to treat them with care and respect as much as you can. Love between you and them might develop over time.