I got mnemonica down pretty fast and reliably by using a multi-pronged approach. I used the Major System (https://litemind.com/major-system/
no need for me to type it out here) to make a peg system for all the cards and all the numbers. Cards are first letter of the suite into the major system peg. Then I also used the story system to make a sequential throughline using my card pegs as images. You could use the memory palace too, I didn't because I already knew my pegs, and peg is actually better than palace if you need to know them from a random number instead of only in a sequence. And then after that, I started drilling it into memory the same way we play scales or learn times tables, doing it both thinking of my pegs, thinking of only the card pegs, and trying not to use the pegs at all, making sure to go from numbers to cards and vice versa. (Make a flashcard deck with numbers). One of the keys to getting into instant memory is breaking the flashcards into chunks. We don't learn all our scales or times tables in one go right? So I spent 20 minutes doing just 1-10, or 11-20, or A-K of clubs, etc. I also did this turning them face down on the table so I got a visual snapshot of the sets of five in a row for 1-5, 6-10, etc. I also spent time making sets of "just the hard ones" and drilled those way more. Basically you want to rely on instant memory with the pegs as the fallback. The nice thing about building your peg system is that once you have it, if you blank out, you can go to pegs and have the card in another second or two, and ... you can use those pegs for plain old memory tricks! And, when you have the pegs as your reliable check, you can drill "scales" anywhere. My dog's new name should be mnemonica.... lol
Keys for working on long-term memory (I do this *a lot* as a jazz musician, we have to memorize new tunes constantly):
- work in 20 minute chunks
- try to memorize the same thing as many ways as possible, building connections between them all
- you want repetitions of the *act of recalling*. Reviewing while looking at the answer does very little compared to the hard feeling of making yourself recall
HTH. Mnemonica is super fun.