Experimental designs are a set of procedures that you plan in order to examine the relationship between variables that interest you.
To design a successful experiment, first identify:
A testable hypothesis
One or more independent variables that you will manipulate
One or more dependent variables that you will measure
When designing the experiment, first decide:
How your variable(s) will be manipulated
How you will control for any potential confounding or lurking variables
How many subjects you will include
How you will assign treatments to your subjects