Building a robot usually starts with an idea about the mechanics. What is the mode of propulsion? How many motors do you need? A good way to make the robot's movement dynamic and smooth is to use springs to dampen and return limbs to a center position. The idea with this robot is to alternate two rotating feet contacting the ground by shifting the weigth of the robot, so three motors, one per feet and one to move the upper part of the robot. With some added mechanics, this could also be achieved with just two motors. For prototyping, adding a motor is usually faster than developing complex mechanics.
How to BEAM: robot electronics.
Once the mechanics are done, the motors need to be activated in some order to make the robot move. With BEAM style robots, this is usually achieved with a nervous net, which consist of a number of oscillators, also known as a central pattern generator. One two node oscillator, or bicore, can be used to generate the signals needed to move a single motor back and forth. The timing is set with resistors and capacitors, and additional resistors/caps connected/disconnected with, for example, a whisker-actuated limitswitch, can be used to affect the timeconstants. More nodes, neurons, can be chained to add delays or more complex behaviors. The signals from the nervous net are fed to an H-bridge or similar motor driver circuitry.
How to BEAM: robot aesthetics.
Aesthetics is an often overlooked aspect of robot building. Until self-replicating robots become a reality, the only way for robots to reproduce and evolve is through smartish apes with screwdrivers and such. Making the robot look interesting increases the chances that someone else gets inspired to make more robots!