Ok this is to make a quick and simple window. In this I’m doing two windows at once with the same height but they can vary in width. If you need to do a bunch of different height windows I suggest after reading this to seperate the wall sections into different seperate mesh so you don’t get tons of edges or weird triangles. To seperate out walls, select all the parts of the wall you want out, hit P and now that section of wall or mesh is a seperate object and you can slice it up accordingly.
Select the wall for your 2 windows. Make sure you are in wireframe mode (z). Press B to and draw a rectangle around the vertices you want to select.
Press K. Select Knife midpoints. Somewhere outside the wall in the blank area LMB click once. You will notice when you move your mouse it has a yellow line attached to it. Click again on the other side of the wall so the yellow line crosses it.
Now with that new cut selected and highlighted in yellow Press G to move your cut along until it lines up nicely with the side of your window in the layout. Select everything again (a) and cut again like you did before. Again move this new cut along the wall to the edge of the window. After you have made 4 cuts and placed them along the wall in the window position move on.
Go into the side view (1 or 3 on numpad). Press CTRL + R. Mouse around the edges near the middle until you see a purple line going across the side of your walls like in the picture below. Click once and then drag this newly highlighted line up to the top of your window. Repeat this CTRL + R process once more to do the bottom part of your window. If it didn’t let you go high or low enough just hit G and then Z to move it up or down however much you want.
Now the fun stuff. Change to face mode first. I put a red arrow to the triangle that you click or press CTRL + TAB in EDIT MODE (tab) and select FACES. The 4 faces I selected, you only see two because wireframe is turned off. You can see all four faces from the top view in the next picture. Make sure you select all 4 faces, the inside and outside of the walls.
With all 4 faces selected and only them selected goto the TOP view (numpad 7). Press Z for wireframe (arrow on the left shows it). If your background image is too bright to see your faces turn it down a little (top right arrow).
Now. While in top view press E for extrude. Right after pressing “REGION” press 0 and hit ENTER. You now have extruded the windows in place, but you will see why in a minute.
You are now going to scale it so the window goes flat. Press S for scale. After pressing S press Y (or X, get that greenish line to go perpendicular to your windows). Next press 0 and hit ENTER.
Last couple things to do before you look at your finished window. In Mesh tools as before hit REM DOUB to kill the double faces we just made. Next change back to solid view Z. Lastly if everything went well press X and select faces (this deletes faces. )
You should have a nice clean cut out for your window and if all your windows are the same height you only need to cut them out from the top view since they are already cut out from the side.