The top knot will rest on the upper region of your head while the lower strings tie around your neck. The front of the mask should cover from the bridge of your nose to mid-neck.
If your T-shirt is too thin, you can insulate with a paper towel or insert another piece of fabric (AKA that excess fabric you cut earlier!) between the mask and mouth. Again, sanitise your mask as often as possible and review safety protocols on how to remove your mask.
Image Source: POPSUGAR Photography / Emily Weaver