To add a variation to a new or existing product, click the Store Tab then click the Products Sub-Tab on the left hand side of the screen. Then either select the product you are trying to add a variation to or click Add Product if you are making a new product altogether:
Once you are in the product sub-page, scroll down to the bottom and click the Edit Variations button:
Next, click the Add Variation button, and you will be prompted with a field where you can enter the Type/Category of Variation:
Once the Variation Type is created, you can input as many individual variations you would like to have available.
Just as well, you can set more advanced settings like different pricing, SKU, weight, inventory. In order to do this, you can just select the Advanced checkbox next to the variation.
Below is an example of multiple variations with advanced settings:
