Many people state that they wish to develop assertiveness skills. That is the first step. Wanting to develop the skills is one thing but doing the actual work takes a different level of commitment.
The first step is to admit the problem and decide this is a skill worth having. Many people never reach this stage so getting to this point is essential. After admitting to the problem, it is essential to identify the specific reasons you have arrived at this stage. Often people mistake symptoms for the problem. For instance, you can hate a word people call you, but in fact, the problem is not the name calling, but the lack of respect that others have that leads them to feeling that name calling is alright. If people are taking advantage of you, it is not often the specific acts as much as the lack of respect that most likely annoys you. Without understanding where you lack assertiveness, it is hard to correct the problem.
If a specific behavior has been continued for a long time, do not expect to change the behavior without assistance. Read, educate and learn how to fix it. The only way to change behaviors is to substitute new healthier behaviors. In order to do that, you have to learn new behaviors to use. Otherwise, you are doomed to continue repeating old, failed behaviors. If reading is not enough, consider getting professional help to work through the underlying issues that may be holding you back from learning better behaviors.
Believe in the right to be treated with respect. This is at the heart of assertive skills. When you believe you're owed respect, you are more likely to insist on it from others. Treat others with respect. People are more likely to give respect when they are receiving it.
Identify the people that are providing assertive challenges. List the behaviors and try to understand what is behind those behaviors. Is the person showing a lack of respect, an attempt at intimidation or anger? Look beyond the act to help decide on options to handle the person's actions. Decide on a plan to address those challenges.
Be firm and respectful. Hostility is only met with hostility. Anger is met with anger. Choose an action that allows restraint and self-respect, but makes it clear that intimidation and humiliation will not be tolerated. This is not the first time this behavior has occurred so there is time to plan an assertive response. It is time to change the nature of the dialogue to utilize the new language and behavior learned.