There are lots of tutorials out there but these two (which use similar methods) are the easiest and fastest I’ve seen. Both are washable/reusable. Both can accommodate a filter if you want additional filtration on top of the resulting 12 layers (I’d cut a piece of a standard HEPA filter vacuum bag for a filter). No scissors or sewing needed, just a bandanna/scarf and 2 rubber bands/hair elastics.
Note: HEPA filters are OSHA certified to filter out 99.97% of airborne particulate matter... the coveted N95 medical grade mask typically filters out only 95% of particles. If you decide to cut up a home maintenance filter to use as a filter in your face mask, make sure that it is made of 100% polypropylene (plastic), which is used in N95 masks. Vacuum cleaner bags should be OK, but furnace filters and other filters may be made of borosilicate (paper reinforced with fiber glass) or other materials that are potentially harmful to breath through. Also consider inserting a wire (e.g., pipe cleaner, sturdy twist tie) at the top of the mask so that you can form fit it to your face/nose.