I can try to get you searching in the right direction. I do small mods. Mostly graphics to add some clan propaganda throughout the game. I've currently been developing a very small clan pack that would overlay and work with most popular packs. I don't know if many clans would be interested and fear of ridicule has kept me from pursuing it on these forums.
As stated, you can do a lot of cool stuff with XVM including the OTM's. Some things you need to look into for creating your own custom content include the following:
For basic skinning and changing a bunch of in game images, you need photo editing software that can work with dds images.
Some images and much of the in game interfaces are embedded in flash files. So you will need to decompile the flash, make your edits and convert them back. This is where knowing action script comes in handy if your are really trying to be creative. I do not know much about action script and that's where I hit roadblocks when ripping apart the flash files. Changing embedded images is what I'm decent at.
The rest of the games function use encrypted python files. So you need to decrypt the python in order to see an editable script. Again, python is not something I've bothered to try and learn. I can decrypt it but that's as far as I've went.
If you gather all of the software needed, you need to prep yourself a project directory to extract the games content to. Within the res folder, you'll see many pkg files. Those are zip folders. Extract what you want but make sure to preserve the path structure. Your mod will never work if it's not in the correct folder.
If you find the needed software and you get things extracted, start trying to edit something simple for your first test. If you get something working, grab an install creator to make your mod look professional and roll on. Packs like OMC and Aslains use an installer where you have to script in your install instructions. The end result is cool though. I use a wizard type installer to keep things simple but doesn't allow for selectable options/preview that everyone loves.
Sorry for being so vague with some of this. I really don't want to toss out a solid step by step guide that would have inexperienced users breaking their game every 5 minutes. Do a bit of research on some of what I touched on and you should find things to help you along.
For most of us, modding is too much of a hassle and best left to the pros. They put in a lot of time and effort to make our game play more enjoyable.
Perhaps someone who can actually do custom content and knows the more advanced stuff can help you out a bit more.