How do I do this? It's not Rocket Science. I think in terms of mental pictures and let those images represent what I wish to remember.
I ponder in groups of seven. To me that is one picture. That mental impression could be Harry Potter's blue Ford Anglia or Paddington Bear meeting the Queen. Anything you dream of.
Then this illusory madness is associated with numbers. A private language only my mind can see. Ninety nine is an Ice Cream, seventy three is a Red London Bus and ten will always be Larry the Cat. So if the next Pi numbers were 2107399, I would see two Larry the Downing Street Mouser Cats on a Red London Bus fighting over an Ice Cream. This story line to collide with the next draft of my imagination. It works!