Setting a Beginning Balance
To set the beginning balance on a new item for simple items the easiest method is to go to the inventory tab as you are creating the product and select one or more stores and click the Set Beginning Stock button.
For Configurable Items you can do the same - but you have to go to each simple SKU one at a time to set the beginning stock. A faster way for configurable items is to go to Inventory >> Adjustment to inventory >> New Adjustment - use reason code Increase Stock add the configurable SKU and use the 'Add All' option to add all the simple variant SKUs. Enter your beginning balances and save the adjustment as 'Complete'