This tutorial assumes you already know how to texture brushes, add entities to a map and edit the properties of entities.

First we need to make a brush that covers the gap you have made for the window, then apply the toolsnodraw texture to the whole brush, next we need to apply glass/glasswindowbreak070a to one of the faces of the brush.

Now select the brush and tie to entity ( CTRL + T ), open the properties for the brush. Using the drop down menu scroll down until you find func_breakable_surf and click apply then close the properties window. 

Now add a env_cubemap entity near the window brush (not needed but gives the window a nice shiny effect) open the properties for the env_cubemap and under key values click brush faces, next click the pick button, then click the window brush in the 3d window. click apply and close the properties window. 

Now all you need to do is compile it

That's it for this tutorial 

Hope it helps

Welshy