VMT $model

The VMT command $model flags this material for use on models rather than brushwork. It is necessary for all materials applied to models.

Remember that the shader type for models is always VertexLitGeneric, but UnlitGeneric, Refract and some other shaders should work too.
Note that VMTs for models should be placed in the game_directory/materials/models folder relative to game_directory/models/your.mdl.

It's also good form to throw your .TGA in there as well, to make it easier for modders to create new skins for your model.

Try to keep the skin for each model entirely in one texture; using multiple textures on a model is a resource hog.
See also Materials for models

VMT syntax

"$model" 1

The QC command $model specifies a reference SMD file to be used as part of this model.

Used mainly for characters when the model needs complex parameters like eyes, flexes etc. In other cases, use $body.
See also: Qc eyes (helpful SDK tool) and Eye Position Setup.

$model "<name>" "<reference_model>.smd"

where

<name>: can be any thing as it is only used in HLMV.
<reference_model>: is the path to a reference.smd, relative to the current path.

$modelname "weapons/shell.mdl"
$cdmaterials "models/weapons/"
$model "Shell" "shell.smd"
$sequence "idle" "shell.smd"

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eyeball "<name>" "<bone_name>" <X> <Y> <Z> "<material_name>" <diameter> <angle> "<iris_material>" <pupil_scale>

<name> Name of eyeball, used to match eyelid rules.
<bone_name> Name of bone that the eye is parented to, typically the head.
<X> <Y> <Z> World location of the center of the ball of the eye.
<material_name> Material name to use when searching for vertices to consider as the "white" of the eye <used in dynamically texture mapping the iris and cornea onto the eye>.
<diameter> Diameter of the ball of the eye.
<angle> Default yaw offset from "forward" for iris. Humans are typically 2-4 degrees wall-eyed. Not setting this correctly will result in either your characters appearing cross-eyed, or if you've compensated by misplacing the ball of the eye, them not tracking side to side. <iris material>Material to use as the iris texture.
<pupil_scale> World scale of the iris texture.

eyelid "<name>" "<expression_file>" lowerer/neutral/raiser <frame> <height> split <distance> eyeball "<eyeball_name>"

<name> Base name of the eyelid flexcontroller. This is combined with the "lowerer", "neutral", and "raiser" options to build the piecewise blend positions of the eyelid.
<expression_file> The name of the file to find the morph targets.
lowerer/neutral/raiser <frame> <height> One of the three options, the frame number in the <expression_file>, and the distance above or below the center of the pupil that polygon edge of the eyelid crosses the white of the eye.
split <distance> Where to start, on the opposite side of the "0" line, for finding vertices that belong to this eyelid morph target.
eyeball <eyeball_name> Name of the eyeball that will drive this eyelid's flexcontroller.