By Transactions 

ADD: Every time you create one of the transaction as follow, you the item quantity that is used will be added to your inventory.

Received Item
Customer Credit Memo

DEDUCT: The same way on the following transactions, the quantity will be deducted.

Quick Sales
Vendor Credit memo

Manual Adjustment

For the First time Adjustment when you have inventory from before, it is best to use "Initial Quantity on Hand"

For Further adjustments

Use "Single Item Inventory Adjustment" or "Bulk Inventory Adjustment"

Single Item Inventory Adjustment

Bulk Inventory Adjustment

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