What is the best practice for creating metafields?

I had someone ask me about transferring custom fields from an app to the new Shopify metafields. I am wondering what is the best practice to add metafields.

  • Right now I see the definitions section in the admin settings which I could use to create some metafields, but it seems very limited.
  • I also see the POST request way in the docs that shows sending in some JSON.
  • The other way is with the admin/bulk? page and putting the metafields as parameters in the url.

What is the preferred, best-practices way?