How can someone open themselves up to be vulnerable in a relationship?

Being vulnerable is a very good concept, however, it can be hard. How can someone open themselves up to be vulnerable in a relationship?
In this case, I defined "relationship" as a romantic relationship. Dr. Ellen Wachtel gives several tips about opening up and being vulnerable with your partner.
First, and most importantly, you must accept who you are and your faults. Once you accept yourself, it becomes possible for others to accept you as well.
Do not "idealize" your partner. Accept that they are human, they have faults, and they make mistakes. This will allow you to gain a sense of togetherness. You will be more willing to open up, and have less fear of making mistakes. Once you realize that no one is perfect (even the people you love), you will be more willing to be vulnerable.
In addition, trust yourself and your decision to be with this person. Trust that they will accept you and love you for who you are. You must be confident in your decision to be with this person. Once you know that you care for this person, and they care for you, you can be sure that opening up will not cause them to run away. If you trust yourself, and trust your partner, you will be willing and able to open up about fear, feelings, stories, quirky facts, expectations, and open with all other things. You will be vulnerable.

"We Love Each Other But...". Dr. Ellen Wachtel. Golden Books. 1999.