Happy Friday --let's talk about custom modules! The ability to create custom modules is one the most powerful features of Drupal. Most of Drupal's functionality is supplied by modules just like the ones you can make yourself. There's literally nothing that can be done in Drupal that can't be done with a custom modules. So you might be surprised to learn that my advice is:
Don't do it!
Or at least, try to see if it's possible to get the results you seek without building a custom module. While it's often relatively easy for a programmer to achieve a desired effect with custom programming (especially if they've been recently working with some of the relevant systems) it's much harder to maintain custom code than it is to upgrade Drupal core and major contributed modules. Carefully consider whether there's a way to get what you need done. (If you do conclude that you should create a custom module, I'll have suggestions later on about how to minimize your pain.)