Check out some DBT skills. They go through interpersonal stuff like that from a basic level.
DEAR MAN is a good one.
Describe the current situation (if necessary). Stick to the facts. Tell the person exactly what you are reacting to.
Express your feelings and opinions about the situation. Don’t assume that the other person knows how you feel.
Assert yourself by asking for what you want or saying “No” clearly. Do not assume that others will figure out what you want. Remember that others cannot read your mind.
Reinforce (reward) the person ahead of time (so to speak) by explaining positive effects of getting what you want or need. If necessary, also clarify the negative consequences of not getting what you want or need.
Mindful keep your focus on your goals. Maintain your position. Don’t be distracted. Don’t get off the topic. Speak like a “Broken record.” Keep asking for what you want. Or say “No” and express your opinion over and over and over. Just keep replaying the same thing again and again. Ignore attacks. If the other person attacks, threatens, or tries to change the subject, ignore the threats, comments, or attempts to divert you. Do not respond to attacks. Ignore distractions. Just keep making your point.
Appear confident, effective, and competent. Use a confident voice tone and physical manner; make good eye contact. No stammering, whispering, staring at the floor, retreating.
Negotiate be willing to give to get. Offer and ask for other solutions to the problem. Reduce your request. Say no, but offer to do something else or to solve the problem another way. Focus on what will work.