How to wear a face mask

As a result of the COVID-19 pandemic, folks are starting to wear face masks in public to help inhibit the spread of the virus (which spreads through respiratory droplets). The guidance from the CDC has long been “only wear a mask if you have COVID-19 and are showing symptoms”, but considering mounting evidence of asymptomatic and presymptomatic transmission, “assume you have it and act accordingly” is the best practice for helping inhibit spread—and acting as if you have the virus and are spreading it implies wearing a mask.

Update April 19, 2020: CDC now recommends a “cloth face covering” to help prevent the spread of the virus. They have instructions for creating such a mask, including a no-sew method if you don't have a sewing machine.

Buuuuuut, a lot of people wear their masks incorrectly, for a variety of reasons:

Many people simply aren't familiar with wearing a face mask, and don't know how to do it properly.
It's harder to be heard when talking, so you have to speak up and enunciate, or else drop the mask.
You can't eat or drink while wearing a mask.
Facial expressions are harder to read.
Lip-readers can't see what you're saying.
Masks with elastics lose elasticity over time. An expired or worn-out mask will droop.
And it's hot and moist under there, so some people will drop their mask for comfort reasons.

But all of these compromise the effectiveness of the mask. To use a mask effectively, you must wear it correctly and consistently—even when doing so is inconvenient. And when you do have to take your mask down, put it back up as soon as possible and then wash your hands!

Correct mask-wearing techniques

Your mask should always cover your nose, mouth, and chin.
Most “surgical”-style masks (often called “medical” masks or just face masks) have two ear loops. Some may instead have two head elastics, like an N95 mask has. Homemade masks may have straps that need to be tied.
If your mask has head elastics, the upper elastic should go around the crown of your head, and the lower elastic around the base of your head. Don't leave the lower elastic hanging!
If your mask has straps, you probably need to tie the two upper straps to each other and the two lower straps to each other, both behind your head. (Your mask may have come with instructions on how to fasten it.) If you're doing a shoelace knot/Ian Knot, pull the half-hitch tight—it'll relax when you finish the knot, and you don't want it to end up too loose.
Many “surgical”-style masks have several pleats running from one ear to the other. These are meant to be pulled taut in the middle, so the middle of the mask is taller (from nose to chin) than the left and right ends (from cheek to mandible). Do this just before putting on the mask.
If your mask has a flexible metal nose-piece, the first thing you do after putting on the mask for the first time is bend the nose-piece to fit your nose. This will hold the top edge of the mask closer to your face, limiting the volume of air that can bypass the mask.
Elastics wear out. If your mask has started to droop or the elastics are visibly worn, replace it with a new one.
You may also want to look into wearing a mask strap extender or twisting the ear-loops to hold the mask more snugly against your face.
When you need to eat, drink, take medicine, or otherwise put anything into your mouth, you'll of course need to move your mask out of the way. After you put it back, wash your hands.
Don't fiddle with your mask in any way while wearing it. If you do, immediately wash your hands.
After you take your mask off when you get home, wash your hands.
By the way, are you washing your hands correctly?

One more cautionary note

Wearing a mask does not exempt you from your other duties:

maintain two meters' distance from other people
avoid unnecessary physical contact
move meetings, chats with friends, D&D games, etc. online if possible
don't go out except for essential errands, work if your work is essential, and exercise/sunshine
wash your hands before touching your face, before eating, and after returning home

All of these things together stack up to a potent defense against the spread of the virus that can help flatten the curve. Doing only one of these things does not mean you should throw the rest out!

Illustrations

Some of us are visual learners, so here are some illustrations to visualize the right and wrong ways listed above.

Correct

Correctly wearing a mask with ear loops.

Only correct in the privacy of your own home

Not wearing a mask.

Please wear a mask whenever you go out in public.

Incorrect examples that I have seen (in person or in pictures)

A mask being worn over the mouth and chin only, with the nose exposed. A mask pulled down below the wearer's face. Possibly done “temporarily” at some point for some purpose, but the wearer should have immediately put it back up and then washed their hands. A mask with head elastics, being worn over the forehead rather than the nose and mouth.

Wearing a mask over the nose and mouth, but above the chin (e.g., because you didn't pull the pleats taut). A mask with head elastics, with only the upper one being worn, and the lower one hanging down from the unfastened bottom of the mask.