Don't know how. Communicate, plan, and be honest. That's how. Stop being afraid of their reaction or what other people will think. You have to listen to your spouse and they should listen to you. Its the only way you can communicate effectively. Communication includes face-to-face, phone, voicemail, texts, emails, letters, notes, body language, meaningful gestures.
I admit, its not easy getting someone with whom you haven't enjoyed being around to join "your team". Marriage is work. Part of that work is to create intentional time being a couple. Remember those romantic dates that you encountered when you were courting. Well, don't let them be a part of history. Carve time out of your busy schedule, and "get busy" spending time together. Talk over a romantic dinner. Take a quiet walk at your favorite park or in your neighborhood. Go roller skating or putt-putt golf. Either way, it has to be somewhere you can talk. You can't "talk" at the movies though. But you can talk at the flea market!!! Anyways, once you start spending special time alone together (find a babysitter!) you will feel more comfortable expressing how you feel about certain situations.
Remember there is a "way" to communicate. I listed several means to communicate. But I want to give you some additional tips such as you never want to start off with a blaming statement. Use "I" messages and follow them up with what you want from the other person. I know you've heard this before. Its in all the therapy literature/websites. But its true . When you start off with "You make me mad when..." you will get nothing but a defensive response and your spouse will close their ears to what ever your saying before you say it!!! So it takes some clever wording here.
For instance, let's say you felt cut down when your spouse told your stepchild that he could go outside and play even without cleaning up his room as YOU told him. First, wait until you are alone- i.e. that evening, or another day when things are calm and your alone- then began by saying "I felt cut down the other day when I told your son to clean up before he goes outside and you let him go out without cleaning anyways. I want you to back me up when I make a reasonable request to the kids." You gave an "I" statement (by simply starting with I, it gives you ownership of the feeling instead of blaming them for it). And you told them what you want them to do next time. One more tip for communicating wisely is to begin with a positive comment or two before you go to your "I" statement. For example, "I want to thank you for taking care of the yard, I know its alot of work. I like how we can work together keeping our house clean."
Its not always clear and simple, but its important to lay out our frustrations in a reasonable way. If you can communicate, your relationship will do nothing but blossom and be built on honesty and trust. Communicate with your spouse today. And you'll be suprised at their readiness to get "onboard".