Alright, Lets say we are making a new texture for the rifle.jpg thats under models>weapons>rifle.
So first step is to re-create the folders
so make a models folder, then inside of that create a weapons folder, and then a rifle folder. Just like in the Data files.
put your rifle.jpg (has to be same name) in there.
Now go back up so you can see the models folder and make it YOURMODNAM.zip (use a program to do that) change the .zip to .pk3
pretty much tremulous is made so that it goes through alphabetically and if theres things that would be in the datafile, then it uses that instead, so you just have the folders the correct name. and they must be alphabetically after data-1.1.0.pk3
Tips: Only create the folders in which you have files in. no need to create extra
Tip: Only put files that you edit into it, no need to make it a huge file for just 1 modification
Tip:If you have a mod thats named "RifleMod.pk3" and you want to override it then the name of the file must be alphabetically after "RifleMod.pk3"
I had the same problem when I started, They need to make a basic Mod Creating Guide like they have for maps.