The women of Saudi Arabia have very few rights. The situation has improved somewhat in recent years, but they are still for the most part second class citizens. If I was a woman from Saudi Arabia addressing the Secretary General (and my life or liberty would likely be in danger because of it), I would point out the UN Universal Declaration of Human Rights of 1948, and ask that both the UN and its member nations pressure Saudi Arabia to enact reforms so that the country is in compliance with that Declaration.
The first thing you must do is to do some investigative work. Do the women of Saudi Arabia have an agenda? If so where is its concentration? It is important for you to clarify specific points from the general agenda. Also, your memorandum must state whether it's forum is the voice of the majority of Saudi women, and if so it should include a possible remedy. The United Nations is best served when those who seek help can also contemplate solutions.
First of all, what is it that they are writing about? Their social status? The situations they face as a result of their subordinate status compared to the men around them? You might do well to include a specific concern, rather than welfare in general. Think of it this way... if you received an F on a paper, and the teacher's only explanation was that it "wasn't good", would you even know where to start in terms of improving it? Make sure you present a specific, clearly defined problem or greivance, so that the problem appears to be something with which the UN can work. If your complaint is that life is just difficult, well, join the throngs of us who work for a living just to barely make ends meet