A legacy letter is a precious gift to our descendants. Its creation is an important gift to ourselves as well. It differs from a typical eulogy in that it's not just a recitation of your positive qualities. It differs from some definitions of an ethical will in that it is not just a listing of values you want to communicate. It is not merely a family history, although elements of that can be woven in. Rather, I recommend that you view this as an opportunity to paint a verbal picture for your descendants of who you were.
If you choose to write a legacy letter as part of your conscious eldering work, begin by doing your best to imagine that you have only a limited time to live — perhaps one month or six months. This gives a sense of poignancy and urgency to your writing. Then try to get a sense of who you are writing this letter for (besides yourself), as this can help move this process from the realm of abstraction to emotion-infused reality. You might write your legacy letter with a specific grandchild or grandchildren in mind, or address it to some other child who is important in your life. Or you might write to future descendants several generations removed. Once you feel your letter is complete, you can decide how and when to make it available to whomever it is addressed, knowing that you can add to it later or can create an addendum that reflects the legacy of your elderhood. It is often the case that young children or grandchildren will find it a precious gift further down the road. Write about what you would most like them to know about your life: events and people that played the most significant roles in shaping who you became; your biggest challenges and weaknesses and how you dealt with them; key turning points, your most dearly held values; your spirituality; the personal qualities and skills that helped define you as the unique person that you have been.
A legacy letter is not something done in one sitting; rather, it is an unfolding project that is best seen as a piece of your ongoing inner work to heal and honor your past as your unique expression of the universal hero's journey of growth.— Ron Pevny in Conscious Living, Conscious Aging