How do I create Custom Windows and Doors?

There’s a great tutorial on window cutouts here and an excellent breakdown of the parts of a window/door mesh here. Doors and windows are similar in that they both have (usually) 6 mesh groups, but you will need to weight paint doors to enable them to swing properly - here’s a tutorial on that.

Weight painting looks intimidating, but for doors, it’s pretty easy, since you only have to worry about 2-3 bones: the transformBone and whichever bind_door swing there is. These areas may be flood-filled as mentioned in the tutorial by selecting all the faces and hitting Shift-K. The parts of the door that swing should be red (100% assigned), and static parts should be blue. On the transform bone, the opposite is true - red for static, blue for moving.

Hope this at least gets you started!