I have added a custom field via plugin to sign-up and edit pages but how do I hook and process that info on submission from my event_handler.php?

I am sort of learning from reverse-engineering google-location plugin.

My question is in class folder location.php and event_handler.php files how do they work to in combination to render and validate and save data from join form?

Could someone post a tutorial on how to add a custom field via plugin to join and edit forms validate on and save.

Many will appreciate it.