Making straps from a flat bedding sheet

A hem is already sewn into the sheet. 

Go 2x the width of that in to the sheet and cut there:

After that is cut off, fold the cut edge inside like so:

Start sewing:

A short distance from the sewing foot, fold the cut edge in on itself again, like so:

Make sure if does not unravel as you feed it in:

Repeat that for the four edges cut off the bedding sheet.

You now have substantial straps for several masks:

The more substantial these are the easier they are to tie in a bow behind your head.