How do I create a code like pic related? what do I need to know.

just make it impossible but include a couple things that can be deciphered to make it seem possible

I'd like to know this as well. This seems really interesting

The zodiac killer used a few cursory cryptography books to create his code. I'm sure with some research you could figure out which ones

>>7691381
Imo it was way more likely the Zodiac got their cryptography know how from magazines and pulp fiction, just like nearly everything else they did.

Use a https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher

Can be cracked but difficult
https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher#Cryptanalysis

>>7690820
By having autism.