Screencast

Steps

To begin, ensure the box Allow customers to purchase this product when it's out of stock is checked in Shopify for each product and variant you want to enable for pre-order.
In the main menu, click on Single Products.
In the search field, type the name of the product you'd like to enable pre-order for then click the Search button.
In the list of products, click the edit button (pencil icon) to the right of the product you'd like to enable pre-order for.
From the list of variants, click on the variant you'd like to enable pre-order for and edit the settings as needed. Click the Save button when finished.
If you'd like to enable pre-order for all variants, first click on a variant, edit the settings as appropriate, click the Copy Settings to All Variants button then click the Save button.

Enable pre-order for this variant: Turn pre-order on or off for this variant.
Use default settings: If selected, this product will use your Default Settings.
Use custom settings: If selected, this product will use the custom settings you specify below (rather than your Default Settings.)

Button Text: This text will replace the text that appears in your add to cart / buy button on product pages.
Pre-order Message: This text will appear near your add to cart / buy button and is used to provide further information to shoppers about fulfillment and delivery timelines.
Pre-order Message Placement: Controls the placement of the pre-order message. The pre-order message can appear above the add-to-cart button, below the add-to-cart button or only when the shopper's cursor hovers over the add-to-cart button.

Enable Inventory Management: Enables inventory management functions so you can show or hide the pre-order button based on product inventory.

Use Shopify inventory management: Show or hide the pre-order button based on product inventory levels in Shopify.
Method 1: When inventory in Shopify is above 0, your pre-order button will be shown. Once inventory reaches 0, your pre-order button will be disabled until inventory goes above 0.
Method 2: When inventory in Shopify is above 0, your usual add-to-cart button will be shown. Once inventory drops below 0, your pre-order button will be shown.
Make [ ] units available for pre-order: Limit the amount of inventory available for pre-order.

Specify inventory available for pre-order: Limit the amount of inventory that can be pre-ordered.

Pre-Order Stock: Set the amount of inventory available for pre-order.
Out of Stock Message: This message will be displayed below the add-to-cart button once pre-order stock reaches zero.
Show Stock Remaining: Show the amount of pre-order stock remaining below the add-to-cart button.
Stock Remaining Message: Customize the stock remaining message that will be displayed below the add-to-cart button when the show stock remaining option is enabled.

Badge enabled: Show a pre-order badge above the product image.
Badge text: Choose the text to be displayed in the pre-order badge. (10 characters or less is recommended.)

Pre-order Start Date: The day on which your pre-order settings will take effect and pre-order will be enabled.
Pre-order End Date: The day on which your pre-order settings will be disabled and pre-order will end.