The textures I am using are called Portal and Portalnodraw like shown below. They can be found in the Common part of textures.

First I am going to do a door. To make the portal, first with the Portal No Draw texture make a brush coving the doorway. Now select the inside part (the side facing indoors) and texture it Portal.

Before portal texture After portal texture Out side

Now let’s work on a window. This is the same way, but just to fit the window.

Before portal texture After portal texture Out side

Now just copy and past them and put them around windows and doors.

Thanks, Playername