Getting married to someone who has previously married and divorced means that you also take on other relationships, such as those with his children and his ex-wife. There's a distinct lack of social norms when it comes to dealing with a spouse's ex-partner, which can make it difficult to know how to treat her. The situation is especially difficult if she has proven to be spiteful of you and your marriage, or if she doesn't want to accept that the romantic relationship between herself and your husband has ended.
Decide what you want your relationship with your husband's ex-wife to be like; be specific and visualise it. If the relationship is vindictive now, maybe you just want to be able to peacefully communicate with her so you can pick up the kids from her house without snide comments. Perhaps you want to have a supportive relationship where you feel comfortable calling and asking for her advice. When you have an idea of the kind of relationship you're aiming for, you are more likely to act accordingly to bring this relationship about.
Talk with your husband about what the situation currently is and what you want it to be. To deal with the ex-wife, you'll both have to be unified in your decisions and communications with her. If she says or does something that makes you feel uncomfortable, talk to him and let him know your feelings. However, don't try to get him to dislike her by complaining about non-issues or manipulating his feelings about her out of jealousy.
Set clear boundaries for what behaviours are accepted and what behaviours cross the line. Let her know what you are comfortable with and what will not be tolerated. You need to be very concise when doing so and firmly stick by the boundaries you've set in place. For example, if she is constantly calling him, you might decide that she isn't allowed to call him unless it involves the children.
Keep any struggles you might have with her between just you, her and your husband. If there are children involved in the situation, never talk badly about their mother to them or try to get them to realise how horrible she is acting. This isn't fair to them and will only damage your relationship between you and her, and between you and the children. Likewise, don't argue with her in front of them if at all possible.