Navigate to Inventory > Inventory Management > Select Parent Product from the Add New Dropdown. Once you are done adding information, click the blue save button.
Under Parent Product Options, select up to three of the Variation Attributes that this product will use. If you have not already created Variation Attributes, click here for more information.
You can create new Variation Attributes under Settings > Inventory Settings > Variation Attributes
From the Parent Product Details Screen, click the blue Add Variation button near the bottom. Search for any child products that will be associated with this parent product and click the green plus button to add those child products to the parent product.
To add child products to your parent products in bulk, see this guide.
For a video about parent/child products, click here