1. Go to Sales > Orders > Orders then select the sales order to be delivered

2. Click on the Delivery tab

3. Click on the picking slip and pick and pack only the items to be delivered. In this example, only the first products will be prepared for delivery then click on Validate

4. Click on Create Backorder since there will be another set of deliveries.

5. Do the same process for the delivery orders as well.

6. You will notice that there will be 4 Deliveries now on the sales order as the undelivered products created its separate picking slip and delivery order because of the backorder option. 

7. Once the products are available, do the same process for the deliveries to fulfill the order.

You can also watch this video for reference: https://screencast-o-matic.com/watch/c0QqQDVC5yw