Starting at the bottom of the shirt, measure 17 - 20 centimetres in height. Cut an indent. Proceed to do the same thing on the other side. Cut across from one indent to another. This base will become your mask.
Mask tip: If you're in dire need of two face masks and don't care for long tie strings, fold the base in half before proceeding to the next step. At the end, you'll have two masks! I did this and found they work just as well as.
Image Source: POPSUGAR Photography / Emily Weaver