There are three assertiveness skills:
1) be direct,
2) ask for what you want,
3) say no to what you don't want.
For more detail:
No criticism (blaming). Everyone's doing the best they can with what they've got. Focus on the behavior rather than the person. Rather than, "You were way out of line," try, "Swearing at the cab-driver was way out of line."
No contempt (name-calling). My pain is no one else's fault. Instead of "You're acting like your mother," try, "What I need from you is support rather than figuring out what I did wrong."
No sarcasm. Use humor that doesn't hurt.
No threats. Rather than, "I'll leave you if you keep showing anger," try, "I want to negotiate an agreement with you about you showing anger."
OUT OF BOUNDS:
Avoid comparisons. Instead of "Your sister keeps her room a lot cleaner than you do" try, "I want you to keep your room cleaner."
Don't bring up the past. Once something is resolved, it's finished. Once someone is forgiven, it's over, and it's destructive to bring it up again.
RULES OF PLAY:
I keep agreements. If I need to renegotiate the agreement, I do so before breaking it rather than after.
I'm polite. Be respectful, even when I'm angry. Instead of "Shut up and let me finish" try, "Excuse me, but could I please finish?"
I listen closely when it's my turn to listen. I paraphrase, and I check to see if I've heard correctly. "Is that what you meant?"
I'm direct. I say what I mean in simple language. Instead of "I did get kind of wet, and the wind was blowing kind of hard" try, "Next time let's pick a place to meet that's indoors." I speak from the heart. I'm open and available, self-revealing.
I praise my loved one. I express fondness. I point out their strengths. And I accept compliments.
I say what I feel as well as what I think. Saying what I feel has great power. Rather than, "I think you should treat me better," try, "I feel hurt and sad that you don't treat me better."
I agree with criticisms when they're right. I apologize when I've hurt someone's feelings. I ask what I can do to make it up to them.
Complaining is good. I complain about problems when they're still small enough to be solved. If I don't, the relationship is doomed to an inevitable and explosive end. "I miss you," is a complaint. "I miss you, and it's your fault," is criticism. And I accept complaints without becoming hostile or defensive. A complaint is said softly and opens space for connecting.
I ask for what I want. I accept that gestures of love are worth just as much when I ask for them.
I negotiate for partial or delayed satisfaction when I can't get what I want. If I can't get even that, I don't take revenge.
I say no to what I don't want. I gently set and maintain boundaries. Taking care of myself is necessary before I can take care of others. People need solitude, privacy, unshared interests. Members of a couple need other friends.
I stand up to abuse, by recognizing it, stopping it and asking for behavior change.
I work for the win-win solution. I avoid making concessions and feeling exploited. I avoid seeing every disagreement as a contest of wills.
I cultivate resilience, flexibility and adaptability. I remember that some problems are unsolveable.