Songs are added to StepMania in the form of "simfiles" which contain one or more "charts" and an actual audio file. These files are usually grouped in "packs" (which some themes refer to as "groups"). You can make your own charts or download ones created by others in the community. This page outlines some resources to find packs and simfiles.
Generally, simfiles come in a few different file formats:
The scope of this category is to include any resources or informative pages regarding the creation, design, structure, etc. of simfiles. Pages for individual packs and simfiles should not be placed in this category.
First, download your pack from the Pack and Simfile Sources. This should result in you downloading a .zip folder containing the pack/simfile you want. Once the .zip folder is done downloading, you should unzip the file and place it into your Song folder.
BEFORE YOU PLACE THE PACK IN THE SONGS FOLDER: Check to see if your download has a nested folder. This will look like
/Super Cool Original Pack Name/Super Cool Original Pack Name/Despacito
This is a rare occurrence, but it may happen. All you need to do is click on the folder that will directly take you to the songs (In the above example, it would be the second pack named folder), and drag that folder into your Songs folder.
Your Songs folder is located wherever you downloaded ITGMania into.
If you are on Windows, Your Songs folder should default to :C/Games/ITGMania/Songs
If you are on MacOS, Your Songs folder should default to /Applications/ITGMania/Songs
If you are on Linux, Your Songs folder should be in the home directory, under /home/<username>/.itgmania/Songs
After you put your Simfile pack into the songs folder, all you need to do is load up ITGMania and it should load in!
For singular songs, you follow roughly the same process as before, except you can't place the song directly into your Songs folder for it to load in game. You either need to create a new folder to place your song(s) in, or place them in an existing folder. The folder name can be pretty much anything, if it doesn't show up in game, it might mean your song folder contains a character that is not supported by the game engine.