There are a lot of books on "engineering drawing" or "techinical drawing" but I think that'd be a little more in-depth than what you're wanting
#1 - Draw it to scale. Get some grid paper, think about the sizes and make a scale (i.e. 1 inch² = 1 square on the paper). Then draw the transmission or whatever you're displaying according to that scale. If there's a 3" gear, then (using the above scale) draw it 3 x 3 squares. This helps when thinking about how it would fit in a robot (it can't be too big!) and how feasible it is too build (it can't be too small!).
#2 - Use guides. Use a ruler and compass. Trust me, these things helps your drawing more than anything!
#3 - Show different angles. I know you said you can't draw but atleast TRY to show the different angles. This would give people a much better idea of what you're trying to show.
#4 - ANNOTATE ANNOTATE ANNOTATE! - If there's a 3" gear, draw a line with arrows on it (i.e. <--->) and put '3" Gear' in the middle of it. This will help people distinguish the parts and compare sizes.
Just be precise. If you have to, draw different parts in different drawings then trace them all onto one drawing (but still retain the split up one so you can show everyone the different parts).