The easiest way to add “clips” to your mask is to use a fork! Your mask should have come with two little pieces. One side is more open than the other. What you want to do is use a fork to insert the end of the mask’s earloop into the more open side of the clip. Feed it through a bit, so that it goes completely through the smaller sized opening and there is fabric on the other side. Then remove the fork. You can now move your clip up and down to the desired length. Repeat on the other side.
Here is a long-winded video of how to do that: