Steps:

1. Create private APP

2. Add API Permissions for the Themes: Read and Write

3. Download Theme Kit from here: https://github.com/Shopify/themekit/releases

4. Move the downloaded file to C:\Program Files\Shopify Theme Kit folder

5. Add the environment variable

• My Computer -> Properties
• Advanced System Settings
• Environment Variables
• In Path include the theme kit folder C:\Program Files\Shopify Theme Kit

6. To make sure Theme Kit is installed properly type theme version in the terminal

7. Make sure you are using the most up to the version by running theme update

8. To download existing theme:

• Run theme configure --password=[your-api-password] --store=[your-store.myshopify.com] --themeid=[your-theme-id] and this will output config.yml file
• Or, you can Create config.yml file manually 
• Run: theme download

9. Create new theme from scratch: 

theme new --password=[your-password] --store=[your-store.myshopify.com] --name="New Blank Theme"

10. Push updates to your theme: theme watch

11. If you make changes outside of your local environment (directly on Shopify) you have to run theme get and this will download all of your changes to your local machine