Some simple sketches should suffice if you can't get CAD.

As you finalise your ideas, drawing your design in something like isometric after having done various projections can produce something pretty good and easier to edit (an eraser and redraw the part.)

I never try anything in Pro/Desktop (the CAD I use) without having it working firstly in my head and then on paper; if I need maths as well, mathematically and if it's a linkage or mechanism etc, in lego or on incredibots.

In a nutshell, my advice, stick to pencil and paper.