We design by three key themes
Useful
Design with a specific issue in mind
Reliable
Design for longevity and durability
Affordable
Design to lower cost
And we design following four careful steps
I. OBSERVE
Observations are needed to ask interesting questions about the world: why do people have a certain issue? What can be done to help? Look at the world, come up with a question.
II. IDEATION
Ideation takes in the observations and aims to produce new and creative solutions to those issues; here's what I imagine doing better or differently.
III. PROTOTYPE
Prototype something small but meaningful, a really simple model, quick sketch or computer simulation. Test it with people, see what does work and what doesn't.
IV. ITERATE
Iterate your design. Products are never correct the first time around. Change the design, fix to problems and test it again.