A fun thing to do is take a picture and import it. Then you can trace it!
This is best done on some kind of grid background but having a ruler (or two) is usually enough.
One suggestion, print one or two layers first to check the fit. Iterate with that before you print the whole thing.
Another helpful thing is to start drawing things parametrically. This should be familiar to programmers. You're using variables and you want to design things primarily through relationships. This becomes a huge unlock because scaling your parts becomes much easier
I started with FreeCAD a couple of weeks ago. Parametric modeling is pretty hard and a couple of things are pretty hard to understand (no easy reuse of sketches between parts for one, one cannot extrude a binder is another one).
However, without it fine-tuning models for technical use would be untenable.
Unfortunately , refactoring is nightmare stuff.
I definitely don't want to say that it's easy, but it's not terribly difficult. Does take a shift in thinking though, but then it clicks.
For reusing sketches, you can. There's external geometry and subshapebuilder. Doing assembly can be a bit tricky at first.
I'll admit, FreeCAD is a bit tricky if you're coming over from something more professional like SolidWorks or CATIA but it does get the job done and you can't beat the price. It's also really improved over the last two years
https://wiki.freecad.org/PartDesign_SubShapeBinder