I'll provide organized steps I'd take, usually along with how I access the operations: To make it more round, you need more vertices. The roundness is primarily a function of how many vertices you have. Knowing that I'd be entirely satisfied only if I manage to keep the top part at the same resolution and the bottom part rounder. What do you think would be the best approach? knowing that I'd be entirely satisfied only if I manage to keep the top part at the same resolution and the bottom part rounder. Spin: Finally, using vertices I created the edge exactly where I wanted with the right dimensions: spin + solidify + some beveling on the inside (that's the result you're seeing).Extrude: Then I tried one (no cap) cylinder + tried to extrude the top narrower part (but failed once again to get the dimensions right).Subtract: My first approach was to use cylinders to subtract one to another until I got to this shape but never got the dimensions right.

Here are the 3 different approaches that I can imagine: So it works BUT I'm not happy with the results and would love to stick to that project until I am so I'm sure to have learned a lot.

The very low resolution of the print is because I found out that making it too round required too much tinkering to get the size absolutely perfect whereas not round fit perfectly. I followed a bunch of tutorials already and started my first project: what seemed to be a very simple cylinder for my son's marble run. I'm an absolute noob to 3D modeling but that seemed handy since I bought a 3D printer.

Blender modeling for 3d printing