Step 1: The Input
To have an input you will need a sensor to have as an input. Your sensor could be a switch, ultrasonic sensor, potentiometer, etc,.
Step 2: The Processor
This is the brains of the operation. You can use an arduino or another simple circuit that will control your output. This is just simply a circuit that will take the input and turn it into an output.
Step 3: The Output
This uses the output from the processor and uses it to do something. This could be a servo or an led.
Step 4: Example
This is an example you can do if you have the Board Of Education Shield kit by parallax. I used one of their projects as an example.
In the project the input is an inferred detector and uses an arduino to process the signal that comes in when ever a light is shined in and the stops the servo output. You press reset and the servos go until a light is shined at the infrared detector and that is processed and the servos are stopped.