Build a Face

Use Shapes (like a sphere for the eye) to help you follow a curve.

This step only can take you a LONG while if you want to do it right.

Once you have the basic outline, follow the contours of the eyes and mouth to fill; the empty spaces (follow the muscles, radial from the mouth if you want to animate your mouth later on)

There's no perfect way for a spline, try to keep it simple and make sure you only have 3 or 4 vertices per face, to use Surface Modifier.

Once you have the surface created you can apply your texture.

Click to enlarge

Then the spline work can be converted into a mesh
Using the Morpher modifier you can create several key poses and use them in your animation