I'm probably classified as intermediate in 3d design and printing. It takes a while to get it right, there are so many factors that influence the outcome, so in

I'm probably classified as intermediate in 3d design and printing. It takes a while to get it right, there are so many factors that influence the outcome, so initially it might seem like the failures are random. Thingiverse models frequently have issues, but you get used to opening them up, checking dimensions and fixing errors in the STLs. When designing, at some point you get used to the fact that it takes two or three iterations to get it right. I frequently print partial models nowadays (or smaller versions of the final), featuring whatever needs to fit and then going back into the design software and completing the model. Then there's the whole "what's the printer up to" thing, is the PLA not too old, is my printer properly calibrated, did the slicer do its job properly. That just takes time to understand as well (having a quality printer here helps as well). Just keep going at it and eventually it all starts falling into place and you get faster and faster at designing as well as incorporating others STLs into your own designs and printing them.