A chapter on memorising numbers from he book
The Memory Book: The Classic Guide to Improving Your Memory by Harry Lorayne and Jerry Lucas uses a similar technique. They recommend memorising this mapping:
0: 's'
1: 't'
2: 'n'
3: 'm'
4: 'r'
5: 'l'
6: 'sh'
7: 'k'
8: 'v'
9: 'p/b'
To use it, just make images/stories that correspond to the digits you need to remember.
The more absurd the story, the less likely you'll forget.
E.g. pi (3.14159265359 ) could be the images of these objects: mit, rat, lab, Noah, shell, mail, bee. Then just sew them together in a story e.g. someone drops a mit, it lands on a rat, the rat runs up to someone in a lab, it's Noah, he throws a shell, it knocks over some mail which lands on a bee.
I don't use it often, but I like to memorise any 8+ digit door code when checking into a serviced apartment or airbnb, in case I forget my phone and would otherwise be without the code.