Adding bile textures to your survivor mod allows it to be more "complete," as it's something that the vanilla L4D survivors have.
Keep in mind that this only applies to survivors. Don't make bile textures for the infected.

In case you aren't in the know how, VMTs are the files that tells the game what textures to use. They coincide with VTFs, which are just the textures.
For this guide, I'll be using my upcoming Mega Man survivor mod as the example for this.

The first thing is to convert the textures that you're going to be using to VMT and VTF, and making sure everything is the way you want it. It should look something like this when you're done.

After that, you make copies of the VMTs.
If at all possible, avoid copying the VMTs for the eyes. You don't want bile in your character's eyes. It doesn't match up with the default survivors. This is unavoidable if the character's textures are just one file, however.

Anyhow, next you rename those VMT copies as "texturename_it."

And when you're done with that, you open up the "_it" file for one of the textures, and delete all of the text in there. Now you want to paste the following in there-
And then, you open up the original VMT file, and copy the base texture line
and paste it over the one in the "_it" file, and it should look like this-

As for VMT files with extra information. (I.E, "nocull," "translucent," etc.), you copy that/those line(s) as well
and paste it just the same.

Now, you want to double check your "_it" files to verify that you have put the patch in.

Now, we can move on the putting it in the QC.

Open up the QC file (It's named "survivor_*survivor's internal name*") in your editor of choice (I use Notepad ++), and scroll down to where the textures are. After putting in the textures that the model uses, copy the line (Assuming that you have all of the textures on a single line) and paste it in the line below it. Add "_it" to the ends of the textures that have an "_it" file, and it should end up like this-

Something that is super, super, important is that you have all of the textures that the model uses in both lines, even if you don't have an "_it" file for it.
The reason for it is so that the textures line up properly when biled, otherwise it'll look flat out wrong.

So after all of the other adjustments to the QC that you're going to make, save it, and then compile it in the model compiler of your choice. (I use Crowbar), and then test out the model in Half-Life Model Viewer to see that everything is the way that's intended.
To see the "skinfamilies," go to the "model" tab, and change "Skin 1" to "Skin 2"

And while the actual bile probably won't show up, you should see that the textures are still looking right.

Stepping away from Mega Man to demonstrate this, you may have seen a survivor mod that changes expression when biled, may it be anything by Tiny King Trashmouth or my own Ninten and Lucas (but most likely the former), and thought "Hey, I want to do this myself. I wonder how to do it."
The operative word there was "may," but ignoring your obvious "no" for an answer, I'll show you how this is done.
Now the most important thing here for this one section is that the character's facial expressions are done through texture. I'll use Paula to show this off. With this in mind, I would recommend having two separate textures for their face (or in this case, their eyes), one for when they aren't biled, and one for when they are.

When you convert these to textures, create an "_it" file for their "standard" face/eyes/mouth/whatever, and instead of the $basetexture being the "standard" texture, you should have it be the texture for the expression that you want them to have.

And when you check out the model, you should see that their facial expression...
...is different when they're biled.

If you're wondering why all of Paula's textures are named "FitNess_part," it's because it was originally from a Brawl mod for Ness.

One thing to note is that the maximum amount of VMTs that a model can have is 32. So at most, a model can only have 16 VMTs prior to making "_it" files. If your model has more than that, then it probably ain't gettin' bile textures.

And that's it. Hope this helped you with you modding in some way!
Mega Man and Paula have been released.
http://steamproxy.net/sharedfiles/filedetails/?id=974308586
http://steamproxy.net/sharedfiles/filedetails/?id=974298437