Designing a model for printing requires a lot of attention to detail. There is quite a bit to be aware of when designing. Careful planning on how it will print can save a lot of time and frustration. Know the limitations of the machine. How much overhang can be printed without additional supports, and adding subtle inclines, can not only make the print less likely to fail, but it may also make it stronger.
I use Fusion360* for my 3D designs. At the time of writhing this page, a free Start-Up/ Enthusiast licenses will allow you to access Fusion 360 Ultimate. A yearly subscription, after the trial period has ended, is available. You can use this license if you are a small business making less than $100,000 per year (or equivalent), or if you’re a hobbyist using Fusion 360 for non-commercial purposes.
More information about Fusion360 licensing available here.
*I am not affiliated with AutoDesk nor do I offer support for their software. It has treated me well and I want to pass it along.
Next: Slicing the Model