When I was building one of my first structures, I had originally planned to use Grandt Line windows and doors, but they kept getting backordered. They are really 1/24 and I wanted 1:20 in my own size. So, what was left but scratchbuilding?
I thought it would be hard, but it was really just tedious. ;) I bought some assorted sizes of styrene strips from the local hobby shop and cut them all to the same length using my "Chopper II" from NWSL.
Next, I built a jig to position the individual pieces of the window so they could be held in place while the glued dried. I cut the squares from thin plywood and glued them in place on a wooden base. I did end up tapering the sides a bit to allow the windows to come out easier. It would be a good idea to spray the jig with something like clear Krylon to seal the wood. It will make removing the window assemblies a lot easier.
This one is for 4-pane windows:
For mullions I used one long strip for the vertical piece, and 2 short ones for the horizontal pieces.
For assembly, I put the mullions in first. Place all the pieces in and make sure they are the right length to meet the window frame. Then, put the long mullion in first, followed by the short ones glued to the center (long) piece. I then build up the window frame around this. Let dry. Remove.
The assembled window pieces are glued together with a slight overlap and then I frame them with some wider strips of styrene.
I have another one for 6-pane windows:
These windows required some extra framing as they were going on a wooden building.
Here you can see what they look like when painted and placed in the building. I’ve added window shades. I also created my own door as shown here. The window on the second floor is a commercial casting. 
My larger windows fit in a stone wall, so did not require the extra framing.
I also built the doors and the cupola from styrene strips.
Update:
For my latest round of windows, I wanted to have them look more like the Grandt Line plastic windows.
I used .040" x .040" styrene for the muntins, and .040" x .060" for the frame for each window.
I built a small jig to help in the window assembly.
The jig is two pieces of styrene at right angles to each other. There's also a tiny sheet of .040" styrene that's usedto support the other window frame. In practice, the interior frames are laid up against the styrene jig strips. I then glue in the upper window frame.
I use a small square to hold it briefly while the glue sets.
Then the other window frame is glued in place. Once dry, I smooth all the edges and add the exterior framing. The interior framing is .060"x .188".
This shows six windows made with this method. A Grandt Line window is also shown for comparison, as well as the jigs that were used to build them.
