The most common way to cue a specific order to be scanned and packed is by using a barcode on a printed packing slip (aka packing list) that has been printed for the order. This is nearly always printed from the Order Manager, (ShippingEasy, ShipStation, etc) where the shipping labels are also printed.
If you do not currently have a barcode on your packing slip (or if you have one generated by ShipStation that doesn't work) the process for adding (or fixing) it is generally a quick one. You can reference the articles below for more info on the order manager in question.