This is no simple feat but by no means impossible.
Other people have done it, there are plenty of DE or WM out there so there is no reason that you, with patient, skill and lots coffee couldn’t do something great.
Learn the difference between DE and WM.
Test different DE and WM, maybe one does exactly what you want it to do?
Make your own custom DE
Here is a guide to get you started creating your own custom DE
After doing all this you should be better equipped to figure out what you actually want to do.
Fork a project and get started.