Preperation
You are required to download and install a few files that comprise the environment of creating custom shirts.
This should be obvious, but you are going to need the Unity project used to create shirts to create shirts.
You can download the project directly at the repository. Simply click the large "<> Code" button, and select the "Download ZIP" option.
As for installation, I personally have my copy of the project in a directory within my Documents folder, though you can decide to place it wherever you'd like to have it.
NOTE: Try to avoid letting it sit in your Downloads folder. It could easily get lost and perhaps deleted in there.