I am sure this question will irritate many old Drupal developers, but I just need a quick fix. I want to

1. create a brand new custom module by a hook to an existing contributed module.
2. introduce a feature in my new module which is an alteration of a feature (e.g form) in the parent module.

Can someone please walk me through the coding ?

Say for example , I want to create my custom module from the simpleads module (https://www.drupal.org/project/simpleads), and my module needs to alter some forms in the src/forms folder. Thanks