Create a sales order as usual and add items. For each item with different ship dates, click 'Details' and select 'Ship Dates'. Enter your dates and quantities in the grid provided then click 'Save & Close'. You can change the quantities at any time provided the order quantity is not less than what you already shipped.
When you fill the order, you will be shown the Batch Shipping screen. We use this when there are more than one ship date in order to filter what needs to be shipped. Click Process to create the ship doc.