to make an add-on you need a model, i suggest blender, but it might be a little difficult to start off with, so use milkshape if your new. Then you have to export it, don't ask me how, into the game. After that, you move on to scripting, witch is the hardest part yet. Like Regulith said, start by shifting through add-ons you already have and messing around with them. After that i have no idea. This is all just based off of experience, reading(the forums) and Knowledge, big word.
And yes theres all that placement stuff and sizing and, if necessary, bones. But what do i know, I'm just some freak talking to you through the World Wide Web, i could be a stalker for all you know. No I'm not.
