After you've connected your Shopify account to your Partial.ly account, you will see a Create Payment Plan option in the more actions dropdown while viewing draft orders in your Shopify admin.

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After clicking the create payment plan button, if the draft order has no customer associated with it you will be redirected to the create payment plan page where you will need to select or create a customer. If the draft order already has a customer, the payment plan will be created with that customer information.
If you already had Shopify and Partial.ly connected and get an error when trying to create a plan from a draft order, we might not have permission to create draft orders on your behalf. Try to disconnect Shopify from your Partial.ly merchant portal, then reconnect again.

After assigning the customer, you will create the payment schedule. You can enter your own custom terms for the plan, or use defaults by selecting the Apply from offer option then selecting one of your offers from the offers dropdown menu.