To use Printful with Customily for Shopify, the first thing you need to do is to create an account on Printful. If you don't have an account with Printful, you can create one here.

Then if you don't have it already you should add their app to your Shopify store.

Once you have your Printful account created and the app installed, open your Customily app on Shopify and follow these steps.

1. Head on to Start or Store on the left menu
2. Go to Settings and open the Integrations tab.
3. If you have more than one store on your account, make sure you are changing the setting of the correct store by selecting it from the top left dropdown.
   
4. Select Printful from the Select a provider for integration dropdown
5. Click enable integration
   
6. Paste your Printful API Key (also known as API token). Learn where you can find your API Token on Printful here.
   
7. Click Save Settings at the bottom of the page.