When you integrate your shop(s), you will begin to see orders imported from the time the integration was made.
If you navigate to the orders tab, you will see that there are three different options: Active, Draft, and Unsynced.
Your goal (if you are on the pro plan) is to move orders that are in the "Draft" and "Unsynced" tabs over to the active tab. This will mean that all of the items in your orders are synced and all of the necessary printing images are attached.
You will want to start in the "Unsynced" tab.
After all of the unsynced orders have been fixed and the index is now empty, you will now want to move over to the "Draft" tab.
After all of the draft order images have been added and the index page is empty, you can now move over to "Active Orders." You should now see every order in this section, and every order item will now be included in your workflow, including the purchase order list and on the "Items" page under manufacturing.