Click on Store Management (This is the 4th item on the left-hand side menu)
Then click on Products to bring up the products of your store.
Next select the product you want to add a inventory count to.
Click on the Show Advanced option and select yes.
Next under Simple Count Value you will put how much you have of that item and then scroll down and select Save Changes.
Now go back to that product and select it.
This time select Get Simple Inventory and now your count is shown. When you sell the number will go down and if you receive new inventory the number will go up.
Now you have a basic understanding of Basic Inventory!