Being a weaver, tying knots is second nature to me but I realize it may not be as easy for others. Here is a video demonstrating how I tie the waxed cotton cords on my mask. This knot has the benefit of staying put, but is also very easy to untie if you need to make an adjustment and it's the reason I chose this cording.
Sometimes I get asked why I don't use elastic on my masks. I decided against using elastic after my first attempts at mask making. I gave my husband Clay a mask that used elastic. It was tad tight and it ended up popping his glasses off his face. His glasses fell on the ground and broke. More seriously, the father of a friend was wearing an N95 mask made by a commercial manufacturer. The elastic broke on his mask and hit him in the eye. He has since lost sight in that eye.
In my view, the benefits of using adjustable cords is greater than the convenience of elastic. I might lose sleep if I thought something I made ended up hurting someone. No shade against anyone using elastic. As makers, we are all allowed our design choices.