Ok, so I had a go at one of these - and despite the instructions being perfectly clear, I still managed to mess it up. Oh, well, that’s why you make a toile first right?
I now know what I did wrong on this one and can make another - where I will no doubt make a different mistake.
More generally relevant, I can say that if you can manage to actually read and follow the instructions, they are clear and do produce a wearable mask without a lot of fiddly cutting or anything beyond sewing in a straight line.
It could even be hand sewn without taking too long.