Fixed images and added a little more information.
This is a fairly easy mesh tutorial. I used to make all my fursuit heads this way for several years (before I discovered how awesome balaclava is!)
While this tutorial features a canine headshape, this tutorial can EASILY be altered to be any other sort of creature by shortening the muzzle, changing the ears, etc.
This tutorial can also be altered to have a moving jaw, just don't add on a chin strap and make the lower jaw separate and attach either with rubber bands, elastic, brads, etc.
Tutorial - Fursuit Heads
-Several large sheets of plastic mesh canvas (used for needlepoint crafts), lots of hot glue sticks), a hot glue gun (either low or high temp, it really does not matter), some thin (1/2 inch) foam, a thicker chunk of foam (such as the seat cushions sold at JoAnn's), some ribbon, a large-eyed needle (eye wide enough for the ribbon), a marker, sharp scissors
Cut out a fairly thick strip of mesh and wrap it around your head (around the eyes). Get it to where it fits around your head with some space to spare. Aim to make it bigger than your head is- hot glue and fur will shrink the mesh. And even if it turns out bigger than you need, padding a head is a LOT easier than trying to make it larger. So figure out how long the mesh needs to be and sew it together. You will most likely need to attach two pieces, as your typical large sheets of plastic mesh aren't quite long enough on their own.
Put the strip around your eyes again and mark where your eyes are with the marker. Doesn't have to be perfect, I usually just make a dot. Then take off the mesh and cut out large rectangles for eyeholes. Put it back on again and make sure your vision is clear. Also try and get clean edges on all the mesh you cut, this reduces the risk of odd poky bits inside your mask irritating you.
Cut out another long strip. Sew one side to the top of your mask, right above/between the eyes. This will be one of two 'crossbeams' that help make up the top of the head.
Pull the strip behind you to the opposite side of the mask. Find where it is snug, but not uncomfortable and mark it (you will have to wear it for this). Take off the mask and sew the strip to the mask where you marked. Cut off any excess hanging off the back. You should now have something like the picture above.
Now repeat the last two steps, having this new crossbeam go from ear to ear across your head.
Mark off how long the second crossbeam should be and then sew it onto the opposite side of the mask, cutting off the excess. Where the two crossbeams meet, hot glue them together for stability.
Now we need to make a chin strap of sorts. Cut out another strip and sew one end to the bottom of your mask, right before the eyehole starts.
Measure how long the chinstrap needs to be and sew it in place, cutting off the excess. Make sure it isn't too tight!
Now you have an awesome special-ed looking piece of headgear. AWESOME!!
This is where things start getting interesting and you start making the mask actually look like your character. Sew a strip of mesh between the eyes and sew the end to the chinstrap. Bend and cut this piece of mesh to resemble the profile of your critter. The example critter I'm making is a coyote, but this design be easily modified to make another species. For instance, if I wanted a cat muzzle, I would shorten the strip and make it more sloped. At this stage, it's REALLY helpful to have reference pictures of your character, especially front and side views.
Start adding on other features, like the basic skeleton for the ears, and a strip of mesh for the lower jaw, if you choose to make the character's mouth opened.
Again, look at your reference materials to check proportions and shapes.
Now it's time to start patching up the holes in the head. What I do is I take the 1/2 inch foam and push it against the openings from the inside and glue it into place. Then I cut off the excess. It makes for a very smooth curve that follows the head shape well.
Another take on this is to use foamies sheets instead of upholstrey foam to close the holes in the mask. I have never tried it that way, but it looks like it might make for a looser mask, which is great if you accidentally made yours too small!
Patch up the empty spaces on the sides of the muzzle in the same way. Make sure the foam piece is curved outwards so it gives the muzzle roundness. Also, be careful to make sure both sides are symmetrical (that's perhaps the hardest part, symmetry). Also start thinking about what you're going to do with the eyes. I started to cover the eyeholes with a double layer of black splatter screen mesh at this step. I always reccommend painting it black on the inside, because having to look through clear, white, or silver mesh will really hurt your eyes and limit your vision.
More foaming work (also finished putting mesh over the eyes). Use pieces of cut foam to help shape the ears, the mouth, the cheeks... basically, flesh everything out and make it look three-dimensional. Figure out the most important features and make them stand out.
Add on more details, like eyes, nose, and other foam details you want. If you're unsure about how something will look, just try it! It can easily be taken off if you don't like it, provided you didn't use a TON of hot glue.
You can even pin a piece in place with straight pins!
Give the mask some finishing touches, like covering the nose (I do it before furring so any edges will be covered by the fur), finishing eyes, etc. You can also use the marker to draw in where the different colors of fur will go.
Your basic head construction is done! :) I would advise putting it on once more to make sure your vision is good, the mask is comfy, and it looks close enough to your character that you're satisfied.
And there ya have it. :)