I’m making a ceiling for a living room, and I want to make 3 downward facing lights “ambient directional”?. But I want them sunk into the ceiling, not flush with it. I’m so sorry for not knowing what these kind of lights are. I did use the Boolean modifier in blender but it didn’t produce what I needed. So I basically need to know how to make a circular face, on a rectangular face, and extrude it, so it creates a little cylinder shaped cavern. 
2 more alternative :
SHIFT+ALT+S (or Mesh -> Transform -> ToSphere) and type 1
Or remove some vertice of the plane/roof , add a circle inside, select both edge loops (make sure both cirle and plane have same number of vertices)
press CTRL+E -> Bridge Edge Loops
(if there was not same number of vertices you will end with having triangle, may not be a problem depending on what you’re doing)
Thanks for showing me loop tools. I just tried the (Loop Tools Circle) But it really distorts my mesh. I’m still pretty new at Blender. 
So I’m probably doing something wrong.
After looking at the images, It seams like the problem is the face not having enough Vertices. There is only one vertex on the face (Not the cylinder). So what I have to do us Loop Cut more faces onto that face, and LoopTools Circle should work, without distorting the mesh???
Thanks for the tutorial. 
you could also add a circle then use the knife project tool
see wiki for description of this tool
happy bl
