I start with the spline line tool (linear creation not smooth) by drawing the outer and inner shapes of the mouth. Then I join the splines together and convert it into a flat polygon.
Once done you need to show or add the hidden lines like in picture so that the shape is made up of polygons. You can do this by making the correct lines visible under mesh edit and turn them if needed. A easier method is to use a script called mesh tools (Very much recommended) to join the points up to make the polygons.
Next select the outer edge border and use the chamfer tool to create more vertices to play with.
After adding more vertices, start by defining the shape of the mouth. Pull vertices back to round out the mouth shape and the lips vertices forward. Make sure you have two vertices close to each other at each corner of the mouth to help define the shape later on.
More tweaking and shaping. I added more vertices along the lower lip and more definition at the corners.
Hopefully you should end up with something like this. I have added another row of vertices along the top of the lip and chamfered the inner lips and moved the vertices more into the mouth to add roundness to the lips. Now on the eye!
I eye is easier to create and is done exactly the same way as the mouth. You can just create the outer shape then chamfer it into the center and delete the center polygon to get the eye socket with the polygons already defined. Then just chamfer the inner border line a few times to create the eye lid.
After a bit of tweaking you should have something like this. I have added more vertices to create the bridge of the nose. Remember to bend the outer side of the eye back a bit and the inner side a smaller amount back. The top of the eye lid should be more pronounced forward than the bottom.
The nose can be tricky at first to model but start with a 4 X 6 flat plane and pull the polys up into a similar shape like this. Add a extra row of vertices along the bottom of the nose to the corner. Note I have only modeled half the nose. You should duplicate and mirror it as a reference so you only need to work on one side.
Create more polys for the nostrils then select the polygon and extrude inward. More tweaking and adding polys and you should end up with this.
After I have finished modeling the 3 main components of the face I put them together to make sure size proportion is correct and they blend together ok. I will also add the eye ball to the model and make any other adjustments to the eye shape.
The next step is to add the rest of the head, which is started out as an sphere and shaped using FDD's into and head shape. It is important to note that I position the end of the sphere at the mouth so the splines follow the contours of the face.
Back to the face, and I will weld the 3 components together, adding or removing any vertices if needed..
Adding and meshsmooth at this time helps me see where any mistakes might pop up, and helps me refine the detail around the nose, eyes and mouth.
Next, back to the head shape. I extrude the bottom of the sphere a few times to create the neck and tweak the vertices making sure that it will blend in with the rest of the face.
Now start dividing up the head polygons if necessary so that it can be welded to the face more easily, this may require additional segments to be added to the head shape.
Finally weld the face to the head and add another mesh smooth to spot any problems and make any additional refinements to the head and face.
Next onto the ear, which can be very tricky. However, I start with an 6 X 10 plane and shape it into an ear then extrude the shape a couple of times and weld vertices where needed to create something like this below. Reference images of an ear are very useful at this point.
Next is to attach the ear to the head, as the ear has more detail, you need to split a few of the polygons on the side of the head so you can weld it successfully.
Modeling the internal mouth requires half the head to be hidden, and this is created separately before being added to the inside of the head. Not much detail is needed here, and makes it easier to animate the mouth.
Finally I will check the whole model and add imperfections to make the head less symmetrical, like bend the noise a little to the left, moving an eye socket down and tilting the mouth. This all adds to realism and brings character to your model.
Finally an test render of the model, before adding textures and the eyes. I would then duplicate the model and deform it to an flatter version to make it easier to paint the textures on then morph it back into the head shape.