First, you learn by experimenting with different materials, using different processes to manipulate those materials. Once you’ve experience each individual material’s and process’s limitations, you’ll understand which combination will work best to create the 3D mockup that you’re aiming for. It takes time and practice, but that is what is necessary.
We had “Model Shop” in our ID program, where we learned the vacuum former, 3-axis mill, metal lathe, wood lathe, band saw, various sanding machines (your new friends), roto-molder, and many other processes. After completing that, I opted for an elective, “Advanced Model Shop” where the materials and processes got much more complicated. These classes were a lot of fun, and allowed me to create realistic quality 3D mockups in all of my remaining ID studio courses.
Oh, and always wear your safety glasses.