A modern (Korean War era and later, at least) signal mirror has not just a hole in the center but the hole is surrounded by a grid of retroreflectors. You aim the mirror spot at something close by, then look through the hole. The grid makes a very bright spot in your vision where the mirror's reflection is going. You then simply tilt and rotate the mirror to put that spot on your target. 

[![This is a good representation of what it looks like.][1]][1]  

There is no need to put a dot of light on your cheek or on a nearby rock or anything else, unless the sun-to-mirror-to-target bounce makes a very shallow angle, say more than about 135 degrees. In that case the retroreflecting grid doesn't work and you have to use other methods. 

Image ganked from [this flickr page][2]. As it says there, several companies do make "signal mirrors" that appear to have a grid around the hole, but don't actually have retroreflectors and don't make the bright spot. If you buy a US signal mirror on eBay and it says "Mark III", it's the right thing. 


  [1]: https://i.sstatic.net/NCA1K.jpg
  [2]: https://www.flickr.com/photos/signalmirror/28053692656