The idea is that you wear once for up to four hours, then wash it at 60°C.
You must not take a mask off and put it back on.
That means that sometimes you'll need multiple masks to get you through the day, and some way of carrying supposedly contaminated masks home to wash them.
The alternative is to wear disposable paper masks like surgeons do in hospital. The shops are selling them at about 95c each. But you can make a serviceable alternative.
You need two sheets of kitchen paper, a paper handkerchief and a stapler. Also some rubber bands or a length of string that will go round your head one and a half times.
Put the kitchen paper on your work-surface and fold it into three lengthways.
Open up the two sheets and pop a folded paper handkerchief between them.
Take the bottom "third" and fold it back on itself. Do the same with the top.
Turn your assembly over. Fold over the ends and either 1) staple a rubber band into the fold or 2) leave a loop big enough to feed your string through.