A Complete Add-On
Here’s a complete add-on: https://github.com/soflyy/wp-all-import-example-addon
Download the .zip and install it as a WordPress plugin: https://github.com/soflyy/wp-all-import-example-addon/archive/master.zip
Once you activate it, you’ll be prompted to install WP All Import.
The add-on is for importing to various fields in Yoast WordPress SEO, so to see the result of what the add-on does you’ll need that plugin installed.
Do a new import with WP All Import (Download example-yoast-data.csv) – you’ll see the add-on show up in Step 3 of WP All Import.
Fork this add-on on GitHub and use it as a starting point for your own add-on!