Print it out on acid-free paper with a stable, acid-free ink. Have it bound as a hardback, and seal it into a waterproof container. Entrust it to one of your children, tell them to keep it in a safe-deposit box and take it out once a year to share with their children, and to pass it on to their children with the same instructions. If you have it electronically, the absolute best case in 500 years is that it will be a…
But well, there is no demand for tech that will last for a millennium. In fact, people are pushing for degradable tech that won't stay as waste after it stops being useful instead.