Conceptually, FDM 3D printing is quite a simple process: you define a set of volumes in 3D space, then the slicing software takes a cut through the model at ever-increasing heights, works out where ...

[DaveMakesStuff] uses 3D printed test tubes for plants and similar purposes, and he’s shared how to make them on a 3D printer, complete with different models each optimized for different nozzle sizes.