Once you have setup stock control (as per this guide) you will need to repeat functions as you add new products. 

1) Add a new product, enter a SKU code, press SAVE, then click Inventory

2a) On the product screen, click the "Inventory" tab and click "Update to Inventory"

2b) Optional: You can find all products that need this by searching Inventory - Website Product Channels:

Select all - and click "Update to Inventory and Channel..."

3a) Navigate to "MC Inventory >Inventory List" and find your product by searching for the SKU code. Click on the item.

3b) Click "Adjust Stock" and add the required amount to give it a stock level. 

3c) Click "Parameters" and set the low stock levels. This is the amount at which the website will display "low stock" on that item. 

For a detailed explanation of how these parameters work please see "Stock Parameters" Page. 

4) If you would like to do this in large batches follow this guide from steps 7a to 10.