Try looking at code from this site. https://codebase64.org
High level steps for what you want to do.
1) Create your new character set.
2) Write an .asm program and assemble it with KickAssembler. (excellent program to use.) The program will need to do the following.
2a) Load that character set file into memory
2b) Program the VIC II to use that character set.
2c) Output some text to see the new characters.
You should start by just learning how to load files into the C64, then learn how the VIC II memory map works so you can use the characters you have created.
Other great IDE programming tools for programming the Commodore series of computers.
C64 studio
https://www.georg-rottensteiner.de/en/index.html
CBM Prog Studio
I hope this helps .