How can I create a new Ubuntu desktop environment/window manager?
This answer lists a lot of available environments, but how can I create a new one?
What are the steps to create an Ubuntu desktop environment/window manager?
Read this to understand the difference: What is the difference between a desktop environment and a window manager?
Each of these options can be fairly difficult and require expertise. For instance: without thorough knowledge on coding software you are not going to get far.
Setting up your new manager is fairly easy:
This page will teach you how to design your own personalized X (graphical desktop) sessions by editing shell scripts. Desktop environments like GNOME often have their own session managers. These session managers allow you to specify which additional programs will be loaded at the beginning of your session by providing a graphical interface. However, knowing how to create X session scripts can grant you greater flexibility in customizing your desktop environment whether you use GNOME, KDE, XFCE, Openbox or other lesser-known window managers like Blackbox or FVWM. Furthermore, being able to create X session scripts allows you to run a graphical desktop even if the graphical login manager, GDM, is broken or not installed.