How to allow pre-orders for products?

Product Pre-Order allows you to sale your items even if products are out of stock on your store.

Please read full shopify article here which is very well written : Allow Product Pre-Order

1. Authorizing payment for pre-orders
   When selling products as pre-orders, you should not capture payment immediately unless the customer is aware that they are purchasing a product that is not currently available. Instead, you can authorize a payment without capturing the funds immediately. Authorizing a payment at the time of checkout allows you to capture the funds at a later date, when the product is available and you are ready to fulfill the order.
   1. From your Shopify admin, go to Settings >> Payment providers.
      Scroll down to find the Payment authorization section.
   2. Select Manually capture payment for orders:

      

   3. Click Save

    

2. Allow products for pre-order
   1. From your Shopify admin, go to Products.
   2. Click the name of the product you want to make available for pre-order.
   3. In the Theme templates sidebar section, change product to product.pre-order

      :

   4. Check Allow customers to purchase this product when it’s out of stock:

      

   5. Click Save.