Well modelling for 3D printing is as similar as to normal CAD modelling, but like any other machining process you need to decide some degree of tolerance especially when you are designing a parts with int. Since, modelling can be done on many software, but if you are engineer and want to design a part that have movement so you better understand above statement. { Note: I am making a pdf or standard tolerance limit, it is currently under process and will be applicable for all design purpose including for 3D printing. } So Let me answer how easy is modelling for 3D printing, well it is totally depend on what type of model you are making it can be a simple toy or toy with moving parts, it can be jewelry or any engineering / architecture prototype. So, if your designing just for basic application I recommend you try online software like ThinkerCAD, Vectary And BlocksCAd. This are the most easiest software that I found for modelling. The best among this that even I use is vectary s...