I started by imagining the alphabet as a circle, assigning each letter its corresponding number. Then I separated it into 4 quarters, giving each quarter a primary color (red, green blue, yellow).
Having done that I created flash cards showing the letter and it's color so I would associate the number and color with the letter.
At first I got most of them wrong but after just a few runs it got better and better. What helped a lot was to create mnemonic aids for each letter number combination. Like for example, I associate the letter "I" (9) with the Intel i9 Processor.
I felt like the last step was the most helpful, while the colors didn't do much for me.