Criteria, constraints and clear steps
Criteria are what the design must achieve; constraints are the limits you must work inside, such as cost, materials or time.
A worked example
A bench for a school yard might have the criterion "seats four people" and the constraint "built from recycled wood for under $200".
The mistake people actually make
Meeting some criteria is not finishing. Change one thing and test again, and check the order of your steps, because a correct instruction in the wrong place still fails.