Here are a couple of online resources and downloadable items.
Some ideas on the construction and use of designspaces: Designspace theory
The FontTools documentation has a page on how the designspace files work. The designspace specification. And this is a page about scripting designspaces. If you want to do something complicated.
A collection of animated problems often encountered while drawing for interpolation
UFOProcessor slices and dices and generates instances. If you want to integrate designspace stuff into your Python workflow, this can be useful package. UFOProcessor on Github
Lightweight font objects built for speed and math: FontMath on GitHub
This was the first designspace package in Python. Largely superseded by FontTools varlib. But it has some perks that makes it remarkably useful. MutatorMath on Github
Batch is a RoboFont extension to generate binary fonts in batch. It can also generate instances from designspaces. Batch on Github
Some suggestions on how to construct letterforms with overlapping shapes here.