CO is your basic texture, created in Photoshop or whatever your preferred texture editing program is.
NOHQ is the normal map, most likely baked in 3ds max, modo or similar. There are filter (plugins for Photoshop) to do it manually from the texture (NVIDIA do them somewhere on their website, but the results WILL need tweaking).
SMDI is an overall shine/gloss map. The best way I've found to make these is to greyscale the _CO, and adjust the brightness/contrast to make any areas I want to be shiny to become lighter, and any matte areas as dull as possible without losing any texture depth. I then overlay a new layer (filled with R:209 G:24 B:209) with 80% opacity. Within this layer I will also darken areas that are to be matte. Sometimes if fine detail such as scratches need to be highlighted I will then overlay the grey-scaled _CO on top with a lighten blend filter.
AS (otherwise known as Ambient Occlusion or AO) can also be baked in 3ds max, modo or similar 3d editing software.
For SMDI, I find it easier to work with each channel separately, in grayscale. In photoshop, I'll create a layer group specifically for specular, and another for gloss. Usually, I try to keep the gloss channel as low detail as possible, with broad areas of different shades of gray. In my experience, this helps to prevent texel clumping - a side effect of converting to DXT formats, which PAA is in this case. When I am satisfied with both images, I'll create a new FINAL layer filled with white. CTRL-SHIFT-C each group then paste it into the appropriate channel of the FINAL (gloss goes in blue, spec goes in green, red channel is max). Saving for web, PNG-24, tends to cut down texel clumping a bit as well. You can save PNG files as PAAs in TexView2, and it will do the final conversion.
I use xNormal for Photoshop which does the _AS and _NOHQ.
Red Channel always 100% White
Green Channel (Is the “metallic” channel. The whiter the channel, the more metallic an object will be). Put scratches, and grunge into this channel. The more metallic you want, the whiter the scratches are. Adding standard black grunge will give you some actual metal depth, and looks alot better than purely adding it to the diffuse. If you want to do it short and sweet, then hit CTRL-L inside photoshop, and adjust the levels until you get a dark grey object. This channel will make the biggest ingame result, as it also controls how much shine a face has. Think of it like this, the whiter this channel is, the more “chrome” it is
Blue Channel is the gloss. Short and sweet is about 60-80% white. But you can get creative and add white to areas of your model you want to have more gloss. Think of a laminated, waxed wood panel floor.
Make sure your base diffuse doesn't have pixel artifacts when converting it to a SMDI, these will showup ingame and look terrible. Rule of thumb is make your diffuse a “solid” color, no “pores” in the paint so to speak
Always remember you can go back into this map and make channel adjustments until you are happy. And it takes time to produce great results, so be patient and practice.
For AS maps the color channels are supposed to be used like this:
Red = white color
Green = Ambient occlusion texture (greyscale obviously)
Blue = white color
Red = white color
Green = Specular map
Blue = Specular Power map (also known as Gloss map)