There seem to be different stages of "readiness" for creating an ethical will. For me, like I've done with many new things I've considered doing in my life, I started by educating myself through reading about ethical wills -- in magazine articles, Barry Baines book on the subject, and on related websites (see Links). With what I learned, I got to a point of willingness (the "tipping point") that got me to begin writing down questions I wanted to reflect on to create content for my ethical will as well as think through my intentions for completing such a document.
As a long-time (off-and-on) journal writer for much of my life, I felt very comfortable about doing the writing but struggled with scheduling time to do it. If there would have been a class or writing group in town working on ethical wills at the time, I would have joined it to "kick-start" the process. (NOTE: Now, a year later, I'm facilitating such classes to help others get started on their own ethical wills).
Whether you join a class to get started or begin on your own, there are (at least) three approaches to use: 1) start with a blank page (usually only for people who've done lots of "open-ended" journal or diary writing); 2) begin with specific questions or "reflections" exercises and an outline (the approach I prefer); 3) start with a list of statements (already written by someone else) for you to choose from and an outline (easiest and quickest, but least satisfying in my experience). Dr. Baines book and software on his website are the only source I know of for implementig approach #3.
One of the most intriguing ways of getting started that I heard from a man at one of my talks was his plan to start with his family heirlooms. He had several pieces of antique furniture which had been passed down to him by earlier generations of his family and he was the only person left who knew the "story" of each piece. He decided to write about the people he received the furniture from, what he knew about when the antique came into the family, and the meaning each piece had for him. Then he planned to place a copy of what he had written about each antique on the back so his heirs would know its family story as it was passed from generation to generation. What a wonderful way to honor past generations of your family and convey your own spirit to future generations!
Whichever approach you use, I think it's most important to write "from your heart", speaking in your own unique voice. Write like you talk (or tape record what you have to say and then transcribe your words). Be sure to use words that "do no harm". An ethical will is not the place to guilt-trip or attempt to change the "evil ways" of anyone in your family. It helps to approach the writing as your "love letter" to future generations -- a heartfelt blessing for the people you leave behind when you die and for their children and grandchildren. Remember that you are creating a personal legacy of your lifetime.