To add a product variation create a new product or edit an existing one.Scroll to the bottom and click on Add Variation
Add your Variation such as: color, size, weight, package etc. and click Enter or the + icon to add the variation. After that type in all the variation types such as: small, medium, large etc. if you are doing Size variation.
You can create as many variations as you want and types for them. If you want to have more advanced settings like different pricing, SKU, weight, inventory just click on the Advanced checkbox.
Below is an example of cross variations with advanced settings:
