Cutting a round hollow

Newbie builder question.....how do you cut a round hollow in something. Im trying to make a frame for an arched door. Any help would be appreciated.

Let's take the cube for example. In the Object tab of the Edit window there is a Hollow Shape drop-down which by default is disabled. This is because you need to increase the Hollow amount. Once you do that, in the Hollow Shape drop-down the Circle choice will allow you to get a round hollow.

Hope that helps!

Right below the hollow percentage is a pulldown to select the shape of the hollow.

Appreciate the help. )