It's difficult to learn assertiveness from a book or handout. The best way is to join a group or talk to a professional person who specializes in this sort of training. However here are a few pointers which may help.

1. Remember that there are many ways to interact with others. We can inform, explain, discuss or simply have a relaxed conversation. It's often useful to know in advance precisely how you intend to interact. For example if you intend to inform another person of a decision you have made that's one thing. You don't necessarily need to explain it and you certainly don't need to discuss it with them unless you choose to. Having a clear idea of the boundaries which you have set beforehand is extremely useful especially when dealing with aggression.
2. Most people have absolutely no idea what you are feeling inside. They only get what you give them. If you look and sound confident people will believe you are confident. Knowing this makes assertive interactions much easier. Try it and see for yourself.

Below are listed some basic human rights. If you work at maintaining these rights for yourself and for others you will be behaving assertively. Incidentally the more assertively you behave the more assertive you become. What you see is what you get.

I have the right to say "No".

I have the right not to understand.

I have the right to make mistakes.

I have the right to be listened to.

I have the right to have my needs met.

I have the right to contribute.

I have the right to dignity.

I have the right to make my own decisions.

I have the right to consideration from others.