Steps:

Before we create a configurable product we need to create the attributes that will apply to our Matrix product. What do attributes do? Attributes can be used to provide consumer choices such as: colors, sizes, logo, etc. 

They can also be used by consumer to filter products on your web site by: material, origin, fuel, etc. First we need to create an attribute. To do so, let’s navigate to “Catalog”>”Attributes”> as shown below. 

To create a new attribute, click on “+ Add New”. The Attribute List Name is for the Name of the attribute, assign it a name. Field type specifies what type of display the attribute will take. 

Once you have filled in the "Attribute Code", "User Label", select the Role of of the Attribute, whether it is Color, Size or other type, you can also enter some description about the attribute(optional).

If we navigate to the  "Options” menu, this is where we can setup the options related to our attribute as shown below. 

You will need to click on "Add New" button and Start creating multiple options for the attribute.

   In this example, we have three options. The sort starts from 1 and moves up in number. This will define the order in which the labels appear. 

You’ll see that we have the sizes “40”, “45” and "50" setup.  These are the exact labels that will appear on the front end and the user can choose between. Once you have finished creating the amount of labels you desire, click on “Save” to complete the attribute. 

Additional article about >> Setting-up Attributes for Matrix Products