Let's say you want to have a product with the "size" and "color" attributes separately.

In our case we'll use the default product product type from admin/commerce/products/types/product and add size and color as fields.

1. Add the size and color fields to the product product type. Their type must be List (text). Check the "Enable this field to function as an attribute field on Add to Cart forms" checkbox.
2. Create products with a variety of sizes and colors.
3. Edit the product field of the product display content type (admin/structure/types/manage/product-display/fields/field_product) to set the number of values to "unlimited".
4. Create a product display node (node/add/product-display). In the
   "product" field, select the products that should be shown on this node.
5. Save the node and it should work as expected, with select fields for size and color