This never replaces modelling things, or making mock ups in cheap simple materials but these come after the design work has been done on paper.
After all Paper and pencil are cheap.
Step 1: Sketching
In general for most people sketching flat is easier and will do.
A side view, a top view and a front view will show you all round your design
If you can manage it, then a 3D view, (called isometric view,) will give most none engineering people a better idea of your object
Step 2: Sketch book
I have maintained a sketch book for my ideas most of my making life. I quickly found I had more ideas than time to make them and if not captured then ideas were quickly forgotten, although I can often recall I had the idea just not the detail.
These sketches often look muddled but do mean things to me and jog the memory.
I also use the book to explore how things may happen and work.