I'll be making an in depth tutorial for my Sans mask!
This covers your entire head, instead of just your front face, for full coverage.
This tutorial also features L.E.D lights for Sans left eye (which is optional of course)
Let's get started then-

Step 1: Materials

You will need:

-Foam head (must be same head size as you)
-E.V.A foam
-Writing utensil
-Disposable gloves
-Hot glue gun
-Hot glue sticks
-Masking tape
-X-ACTO knife
-White fabric
-Black fabric that you can see through (such as buckram)
-Measuring tape
-Black paint
-White paint
-Contact Cement glue
-Medical mask/respirator
-Black craft foam
-Blue battery powered L.E.D's with switch (optional)
-Low grit sand paper (I used 90)
-Higher grit sandpaper

Step 2: Foam Head Defining

Now you may need to skip this step, $5 foam heads are particularly small.

To increase the size, wrap your foam head in plastic wrap until it is the same size as your own using the measuring tape.
After it is the right size, cover the entire head in masking tape.

Step 3: Pattern

Trace out a pattern and cut out using a X-ACTO knife.
(Now, I ended up reversing the pattern here; the front face pieces are now the back of the head.)

The long stripped pattern runs from the back of the head, to the front.
This makes the rest of the pieces easier to curve on the head.

Step 4: Editing

Note: When a pattern puckers, cut a slit through the middle. You will glue the slit back together to create a curve.

Step 5: Putting It Together

Once you've traced, and cut out your patterns on the E.V.A foam using your X-ACTO knife, glue your designated pieces together.

Note: It helps to label them 'right' and 'left' side for your mask

WARNING: You will be using Contact cement glue, the actual liquid, and vapors can be harmful when inhaled, or touched. Please read through the hazard label on the product before attempting to use it. Wear a medical mask/respirator and disposable gloves when handling. Use in a well ventilated area such as outside.

Spread 2 thin coats and wait a few minutes for it to dry before connecting the pieces.

Step 6: Cutting & Sanding

Cut out the eye sockets, and nasal cavity.

Sand down the edges and seams of the entire mask using a low grit sandpaper
Then, smooth it out with a higher grit number sandpaper

Step 7: Coverage

Coat the entire mask with contact cement glue, and cover with white fabric.
You will have to smooth out the wrinkles, and make a few visible seams. Cut them to reduce visibility.

Then you will cover the eye sockets and nasal cavity with black fabric from the inside

Step 8: Painting & Sealing

Paint Sans' features on (with a reference of course)

You can use any type of sealant, I put on Mod Podge fabric stiffener here

Step 9: L.E.D. Lights

OPTIONAL: I taped a string of L.E.D's onto the left eye
To isolate the light direction, use craft foam in the shape of the entire eye socket, and cut out a smaller hole for light to pass through. Hot glue from the inside.