ArmA 2 Texturing basics
_AS and _SMDI files are ambient shadow and specular textures (the purple textures). In both channels are Red (255,255,255), Green (standard 0,0,0 this channel should contain parts that are wanted to be specular or have effects from ambient light) and Blue (255,255,255). Only green should be edited.
It sort of depend for what you're making the RVMATs. For soldiers you need additional _as texture (you can use _smdi as addition for specular parts in case you have some) and for vehicles, weapons etc. you need to have _smdi texture.
Lets say you are making a weapon (or vehicle) and you need specular map for it (_smdi) you just take layers that you want to show in weapon as shine (whitish parts, any gray gradient above 0,0,0 will have specular power, lower the value lower the specular power on that area) and those that doesn't shine (black parts 0,0,0) and you make background say 15,15,15. Merge the visible layers (make sure its all within gray gradient, results are best this way).
Now you make new picture at same size that has following color RGB = 255,0,255. Go to channels and select green channel, then copy paste your earlier edited picture to green channel. Now when you select all channels again there might be some visible change in whole picture, if not don't worry. Save it as .tga and convert with TexView2.
Make sure you use proper suffix (_co, _ca, _nohq, _as and _smdi are the most important ones) in your textures (it makes your life a lot easier, no money back guarantee). Now you should have three .paa files with names like my_name_co.paa, my_name_nohq.paa and my_name_smdi.paa (first being your tag, second name of texture, then suffix).
Now we finally get to actual part of RVMATs, there is huge load of people that are better familiarized with this than I am, for instance Linker Split.
However my style of doing this is to copy BIS existing RVMAT-files and changes names and values in there to match my need. So basically you just have m16_metal.rvmat from BIS example model release and paste it with your files. Change name of it and open it with for instance notepad so you can rename paths of _nohq and _smdi to your own files. Then save.
Finally open Oxygen2 and apply RVMAT to your model (or part of it is supposed to be if you have multiple textures applied). Select faces to include it, hit E and you should see Material reading there, you should be able to find your own stuff from your own computer
That should do it.
One thing as reminder, in order for textures and materials to show in-game, they have to be P: drive and in their respective folders (folder name will be name of the .pbo file) when you start packing the files.