Product metafields can also be managed from the Shopify adminpanel directly. You don't need a third-party app or chrome extension anymore!
From the adminpage, go to settings (bottomleft corner) and go to Metafields. From there, you can add the metafields to the product and set the desired contenttype (inputfield, image upload, checkbox, etcetera)
You will need to provide a namespace and key, which you can refer to from the code template. For instance:
setting the "namespace and key" field to my_custom_fields.somefield
, you will be able to access it from the product template file (product.liquid) like so:
{{ product.metafields.my_custom_fields.somefield }}
or, of course, assign it like so: {% assign myField = product.metafields.my_custom_fields.somefield %}
The created metafields will appear on the product editpage at the bottom. No chrome extensions needed!!