Experimental design means planning a set of procedures to investigate a relationship between variables. To design a controlled experiment, you need:
A testable hypothesis
At least one independent variable that can be precisely manipulated
At least one dependent variable that can be precisely measured
When designing the experiment, you decide:
How you will manipulate the variable(s)
How you will control for any potential confounding variables
How many subjects or samples will be included in the study
How subjects will be assigned to treatment levels
Experimental design is essential to the internal and external validity of your experiment.