You don't need much to make good masks. Just a t-shirt.

I made a couple of the FU masks lots of folks are making and found that they were very slow to make and that they didn't fit well on your face (meaning you were mostly breathing around instead of through them, which makes them far less affective). These are flexible and form fitting and can be made in one tenth the time. you could also insert additional filter materials in the pocket if you wanted too, but I find three layers of t shirt fabric to be pretty good.

You could also follow these same instructions using a stapler in place of a sewing machine but I have a machine so I'm using it.

Here's a youtube video of me making one -YouTube Link

Supplies

t-shirts
pipe cleaners
sewing machine (could easily use a stapler, safety pins, etc if you don't have a sewing machine)

Step 1: Step 1: Open Up T-shirt and Cut Out Square

Lay a t-shirt down flat and cut up one side and over the the shoulders through the sleeves.
Then cut an 18 x 10 inch square out of it. You'll be able to get 4 of these at least out of a large t-shirt like the one shown.
You could go smaller here - I have a huge head and this fits me. Probably best to have different sizes for different people.

Step 2: Step 2: Cut Strips for Ties

Make a slice into the middle of both long sides about 3/4 of an inch deep. This will determine how thick the ear loops will be.
Then slice from the bottom of that cut down the long sides maintaining that width until you get to about 3/4 of an inch from the short end. Do this outwards from the small slit you made in both directions so you have a strand hanging from each corner.

Step 3: Step 3: Fold in Thirds

Fold the piece in thirds so the four strands end up at the four corners of the folded cloth.

Step 4: Step 4: Sew Sides Up and Add Pleats

Fold about an inch of the sides back under the mask this will help it to seal to you face so you don't just breath around it.
Start sewing down the folded edge, being careful to sew the ties folded back out again.
You don't need to pin the pleats in for these. They aren't fashion accessories so they don't need to be perfect.
To pleat quickly: once you've started the stitch slightly raise the foot of your machine and push a fold of fabric under it. You'll want to do this three times or more. Repeat on the other side.

Step 5: Step 5: Add Nose Piece

Take a pipe cleaner or other piece of moldable wire.
For the pipe cleaners they aren't very stiff so I fold them in half then twist them together a bit for extra rigidity. Depending on what you're using you also may want to fold them over.
Open the flap in the bottom of the mask and work the wire up so its right under the top of the mask (I'm pinching the pipe cleaner in the one image).
Holding the pipe cleaner tight to the top off the pocket run a stitch directly under it to hold it in place.