This is a very quick and easy gamer fashion accessory to make. I'm surprised that Nintendo doesn't sell them already.
I made one of these for my dog. His name is Bowser after all. (This is not an excuse for me to show off my dog in a costume)

Watch the video to get a better understanding on how to do this DIY.

Supplies

White Cotton Knit Fabric
bandanna
Scissors
Sewing Machine
Thread
Fabric Markers
- black
- magenta
- red

Step 1: Step 1: Cutting Out the General Shape

Lay out your white fabric.

You can even use an old t-shirt to make this. Make sure the fabric is nice and smooth on a flat surface.

Lay your bandanna on top of it.

I laid my bandanna at the corner edge so I only have to make one cut.

Fold your bandanna into a triangle.
Cut out the shape of the triangle.

Step 2: Step 2: Hem the Top Neck Line

Fold over the longest side twice to create a rolled him.
Pin it in place.
Sew down the edge of the fold.

Step 3: Step 3: Creating the Mouth Shape and Teeth

Using a fabric pencil draw a kidney bean shape to start the mouth shape.

You want to use something in a light just in case you mess up. Make the shape almost as big as the bandanna itself.

Draw a smaller inside kidney bean to create the lips

Because the first shape is drawn so big, go about 1.5in to 2in inside the first bean shape.

Draw in the teeth shapes.

The teeth shapes should have 4 teeth on the top, and 4 on the bottom. It's okay if these are not perfect. They'll get fixed during the coloring phase.

Step 4: Step 4: Outlining the Line Art

Begin preparations for your fabric markers.

Mine said to shake well and press down on cloth until ink flowed. This might not be the same for everyone so be sure to check what yours says to do.

Using a black fabric marker, go over the lines you've created.
Create more of a sketchy look on the teeth

Make sure to double check that the ink is always flowing.

Step 5: Step 5: Coloring

Use a magenta fabric marker to color in the mouth area.

Do not color in the teeth. It is okay if it's not perfect. In multiple references it's supposed to look a bit messy.

Using either the black or magenta marker, create little marks on top of the teeth near the gum lines

These are dots-dashes on the teeth. It's not a huge deal if you don't have them.

Use the black fabric marker to create a bit of shading on the teeth and inner lip area.
Use a red fabric marker to color in the lips. Make this look as child drawn as possible.