I would add: Don't be afraid to try some practice techniques on simple shapes first. Designing a robot is a huge task and it's so easy to get overwhelmed or frustrated early. The polygon fundamentals are key to understanding the design process and yes, practice, practice, practice. So instead of a full mech - why not design something smaller like a med kit from a video game. When you are done with that, design another one. When you are done with that, try designing an external hard from the year 2086. Etc. Jumping right into a huge build is usually a Herculean task designed to fail. Vitaly Bulgarov didn't start with the GITS Major body first.
Also - even with precise and mechanical models, you still have to rough it in, get the proportions correct and then fine tune the details. I see too many people who want it to be perfect right from the beginning and get every nut and bolt and piston, etc in place right out of the gate - without looking at the overall scale and proportions of it. Think of it like clay, even though it is hard pixels - get the overall form going in the right direction, and then start to sculpt into it.
Here's one of my favorites for inspiration:
And the original piece:
Also like this series:
http://lesterbanks.com/2017/04/modeling-blast-doors-c4d/
e.