Help with HG/SS rom edits

[GiantLivingAnt]

So after trying to contact, uh, a few people I think it's safe to say they either forgot or just didn't respond in general(imagine) so basically;

How would I do something like add a pokemon to the overworld, when you interact with it, it starts a battle with said pokemon? Kinda like Groudon or Kyogre in the Embedded Tower? What are the steps and what tools/resources can I use to help me with this?

(If it wasn't obvious, this is under the implication that I have no, no goddamned idea or experience in rom editing or modifying)