Here is the best article I have seen so far on how to make a DIY face mask. It discusses not only how to make the mask, but also the best materials and design features. It also ponders the limitations to protection. But most important, it describes how you should don and remove the mask to avoid contaminating your face and defeating the whole purpose of wearing a mask in the first place.
There is one design feature missing from the masks described here: they leave an air gap on each side of the wearer’s nose. Commercial masks often have a soft metal wire embedded in the upper edge that is easily shaped to close the gaps and achieve a better seal.
Facial hair reduces the effectiveness of all masks because hair holds the mask away from the skin and leaves gaps where unfiltered air can enter more easily that it passed through the mask fabric. Some designs fix this problem by including a soft wire [pipe cleaner, copper, aluminum] in the top fold. The user shapes the wire to the contour of nose and face.
All these things considered, some mask is better than no mask — if nothing else, it serves to remind you not to touch your face.
If the wearer is sick and doesn’t know it, as all cases are during the 5 to 14 day incubation period for COVID-19, the mask helps to contain the virus. If you are well, and near someone who is shedding virus, the mask may reduce the dose of virus that reaches your mouth and nose. The severity of the infection that develops appears to be dose related. Your DIY mask may mean that your immune system doesn’t have to work quite so hard if you are infected.
If everyone wears DIY masks, the virus must escape the mask of the sick person, travel some distance through open air to a new victim, pass through that person’s mask, and reach his or her nose or mouth.
To make the no-sew masks fit better, salvage the heavy aluminum foil from a disposable pie pan. Cut a wide strip and fold it lengthwise twice to make a bendable strip for the top edge of the mask. This will allow you to shape the top edge to seal over your nose and across your cheekbones.