There really isn't a class that you can take, or a way to mentally prepare for all of the ups and downs that can occur when dealing with your significant others ex, specifically when it comes to their children.
If you are blending a family, then you have exes on both sides to deal with. There are holidays, sporting events, school plays and birthday parties in which you must deal with one another. In an ideal world everyone would put personal feelings aside and do what is in the best interest of the children. That actually happens more than you think but, not as often as it should.
Below are tips that may help you and your spouse deal with stresses of the ex-together
1. Always operate as a United Front! (Extremely Crucial).
Ex-spouses may try to manipulate circumstances if they feel they have a "leg up" over your spouse. (I.E...can be the children, or feeling a sense of entitlement toward your spouse). Communication between the two of you concerning any, and all decisions regarding the ex, children, changing of schedules etc. Being in agreement depletes some of the power the ex feels he/she may have and can relieve some stress from the situation.
2. NEVER, EVER give your ex the impression that you want to do what he/she asks because if you do, you'll be in the dog house.
Your ex will take this as an opening to ask you about your relationship. ("Without really asking)". You may not do this purposely, but this will have a long term effect on your spouse from both your ex and children.
3. If you and your ex have shared parenting, or specific visitation days, when/if possible arrange the dates so you and your spouse have some nights/weekends alone.
Blending a family and dealing with all that comes with it can take a toll on your relationship. It's up to the two of to stay connected and committed to making this work.
4. When planning a night or weekend with your spouse and your ex has a pattern of interrupting, ask a friend or family member if they will be the "go between" in case of an emergency.
Then talk with your children, share you're plans, but no specifics. Let them know if an emergency arises, or they simply miss you, they can contact the "go between" who will then get in touch with you. Even in divorced situations, children want to see their parents happy. The fact that you're sharing your plans gives them a sense of being included. The bonus here... It sends a very undeniable message to the ex that he/she is not welcome. To avoid the "20 questions" your children may have coming, inform your ex that your children have no idea where you are going, so no need to ask them.
5. You should not allow your ex to speak badly about your spouse.
Shut that down immediately!!! This is a most definitely a way of baiting you into a fight. It really doesn't matter what they think about your spouse, or anything else for that matter. You are divorced for a reason. If you do not get along on any terms, then there isn't a reason to discuss anything other than your children. If he/she can't seem to stick to the subject of your children, and two out of three conversations are toxic, you should have a mediator delegate a schedule for you both. If changes need to be made, that individual is to be contacted.
After a divorce many life changes take place, and will for years. Setting boundaries right off the bat is key for your future and future relationships. Second marriages face challenges that most first marriages do not. You not only married your spouse, but in a sense married his/her ex. Do your best to keep your relationship with your spouse, and the family you are working to build first and foremost in your mind.
It's easy to get lost in the whirlwind of crap that may come your way. It's up to the both of you to work together, figure it out and make it work! It most certainly can be done!!