From what I can tell from the Internet not having used it myself, the process goes something like this.
Take the codes and format them like this:
Et cetera, but obviously using the actual codes and names instead of my made-up example ones.
Save them in a .txt file named after the TITLE ID of the game. For Super Mario 3D Land, that'd be "0004000000054000", so you'd name the file "0004000000054000".
Make a folder with the same name as the TITLE ID as well, put the text file in that folder, and put the folder in Citra's cheats directory.