To add a variable product, either edit an existing product or create a new one by going to: Products > Products. You can select the Add Product button or edit an existing product from there. Once in a product select the Variable product from the Product Data dropdown.
In the attributes section, you’ll need to add attributes first before we can create variations – you can use global attributes or define custom ones specific to each product. To use a global attribute select it from the dropdown and then select Add. Make sure to select the Select allbutton to add all of the attribute to the variable product, and select the Used for variationscheckbox to tell WooCommerce it’s for your variations:
You’ll then need to name the attribute, for example Size. Set the values separated by a vertical pipe, in this instance small | medium | large. Finally select the Used for variations checkbox to use these in the Variations section. Select Save attributes!
WooTheme’s Guide: https://docs.woothemes.com/document/variable-product/
