Adding Credentials

In order to access the Google Drive API to read spreadsheets, log into your Google account and go to the Google Developer Console.
From there, create a new project with whatever name you want.
Then, enable the Google Drive API for the project you just created.
Open up the dashboard for the project and go to the Service Accounts section on the sidebar.
Create a new service account. The name and description do not matter, most of the text boxes can be left blank.
Once it is created, it will appear at the end of the list on the Service Accounts page.
There, open up the Actions menu for it and Manage Keys.
Press Add a key > Create new key > JSON. This will automatically start a download of a JSON file. These are the credentials for the service account.
If you lose the key, you can go back to the same page to generate a new one the same way.
See below for the two ways to add the credentials to the repository: The original JSON file or Environment variables

Adding Credentials - JSON File Method

To add the credentials to the repository using the original JSON file, simply go to the root/main folder of the repository and create a folder named private/.
Move the downloaded credentials file into this folder and rename it to credentials.json.
All done!

Adding Credentials - Environment Variable Method

This method is recommended if you intend to keep the credentials private but need to send the files to another location like GitHub or Heroku.
First, set the environment variable process.env.VARIABLE_MODE to TRUE.
Go to the page where you can set the Environment Variables in the selected service and open up the downloaded credentials JSON file from earlier.
The name of each remaining environmental variable to add will be the keys from the JSON file like type, private_key, auth_provider_x509_cert_url, and more.
Add the value of those keys to their respective variables to complete the credential set-up process.

Running the project

Before running the project for the first time, run the command npm install to install the necessary dependencies.
The first step can be skipped every time after that. To run the project, simply run node . or node app.js. Note: The service account email must have permission to read and write to the spreadsheets being used when running the project later.