You can perform attribute mapping under the Attribute Set tab under the Shopify Channel. Before you do so, you need to set up the Catalog Attributes first. Check out our PIM knowledge base on the catalog setup.
After you have created your Shopify channel, by default there would be an Attribute Set named "Shopify" created. You can perform the field & attribute mapping by clicking "Map attributes" under the Shopify mapped attribute set.
You can perform "Sync attributes" to retrieve the default shopify fields, and also use the Add attribute to map the Metafields and Variant Attributes.