That depends on how able you are to use Fusion or a similar CAD program and wherever you are comfortable doing that. If you feel confident in using a CAD program to design your robot, cool but prototyping from material can also work, it does help inform you what the robot will look like in real life and what the potential pitfalls of that design might be, CAD might not immediately tell you this. I tend to sit down with a pen and paper, build the machine from there and if I need complex/accurate parts, then I CAD those (it is what I am doing for my middleweight).