How to Make a Mask

Step 1 (Supplies):
Get some plaster cast and some Vaseline.

Step 2 (Prep):
Cut the plaster cast into strips of different sizes and widths.
Apply Vaseline to the face of your model.
(You will also require a bowl of water, preferably warm for the model's sake.)

Step 3:
Soak the strips of plaster and build them up on the face of your model. No need to do a lot of layers. This is just to make a nice mold to work from.
Let dry for about twenty minutes or until the plaster cast no longer feels like it can bend.
Remove from your model and leave to one side for at least 24 hours to harden completely.

Step 4:
Determine what animal you will be creating. A good way is to ask the model/customer what they want. If they really don't know then ask a few probing questions to determine what animal would suit them best. Build on the mask as appropriate.

Step 5:
Problem solving
Ears can be a challenge. There are many options. You can build them off of the mask you just made, but this poses the risk of making the mask too heavy or difficult to wear.
You could make faux-fur ones, which are usually easy to rig up as head gear. But faux-fur can be expensive, difficult to find and require a sewing machine for assembly.
The other option is to improvise. Dig around and see what you can find. In this case, an old top hat will do.

Step 6:
A solid frame is essential to well developed costume ears. In this case I used wire and some card stock...

*note*
This is messy work. Doing it indoors or out, make sure you lay something down on your workspace to minimise the mess. Unless you have a studio space, in which case have fun, be liberated and make as much of a mess as you like knowing you can shut the door on it when the day is done.

Step 7:
Build up the ears using strips of plaster. Make sure to minimise the amount of plaster cast you're using. It's a lovely durable and easy to use medium, but it can get hefty and if someone is going to be wearing it for a prolonged period you want them to be comfortable.

Step 8:
Painting!
In this case the mask is that of a hare so I used a stiff bristled brush to help create a furry look on the face and back of the ears.

Step 9:
Attach the ears to the hat and viola! Finished product! A lovely hare mask suitable for a tea party.