A quick way to get a local shopify dev environment going:

Installation and Setting Up

The first step is to install ThemeKit.

Then, from your Shopify admin, Apps > ‘View Private Apps’ > Generate API Credentials with “Theme Templates and Theme Assets” to “Read and write” (doesn’t matter what the rest are) > Save > Copy the ‘password’ and store it.

The Environment

If you want a new theme, then:

theme bootstrap --password=[password] --store=[yourstore.myshopify.com]

If you want to use an existing theme, get the theme ID from the shopify admin. Go to Online Store > Themes and select “Edit HTML/CSS” on the theme you want a local environment of. Copy the digits at the end of the URL — this is your theme ID.

Then, configure and download with:

theme configure --password=[password] --store=[youstore.myshopify.com] --themeid=[your-theme-id]  
theme download

Running

Use theme watch to run with a watcher on all the theme files and live-upload to the site.