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- When in a Sale (whether it be an order or a shipment that has a status of open)
- Select which lines of products need stock adjustments done (use the Ctrl button on your keyboard and use the mouse to select the product lines)
- Right click on your mouse and hover it over New
- Further options open
- Click on Stock Adjustments
- A new stock adjustment window opens which automatically adds in all product lines selected from the previous sales window
- Change the quantities if required (it automatically defaults the quantity as the quantity ordered from the sales order/shipment)
Select a reason for the adjustment
Expand title Show me 
- Click on button Finalise
(Note: save will not action the stock quantity adjustment, it will only save what you've done so far on it. Finalise is what affects the stock on hand)