The first step is to set your CFW in your profile. This will allow you to automatically download the cheat code in the correct format.
Then, once you have found the cheat you want, you just have to download it and extract it at the root of your sdcard.
Be sure to extract the content of the archive at the root of your sdcard, something like that
Here's an example for Animal Crossing: New Horizons.
Be sure to extract the cheat on the sdcard and to have a file at the following location :
/atmosphere/contents/01006f8002326000/cheats/F829A27213D46F9F.txt if your cfw is atmosphere.
Double, triple check your Build ID, they MUST match.
Update to the latest version your Custom FirmWare.
Your cheat code should be lengthy. Keep it under ~100 lines and SHORTER than 32Kb
Check for strange characters in the title and keep it less than 64 characters. Remove any characters that aren't letters or numbers.
Every time your game updates, you need to update or get new cheats for this specific release. Cheats are made for a specific version of a game.