Calculations
When you have decided on the basic design for your device, you will need to do the calculations to make sure that the design will achieve the aim: how much force is required, what size parts are needed, and so on.
Make sure that you double-check your own calculations (or get a classmate to do it for you) and include all calculations in your project documentation.
Watch a YouTube playlist of all the videos for the 'Mechanical Engineering Design' lecture series