MagPi tutorial on building a simple supercomputer cluster.
See MagPi #87 for full details and instructions.
https://raspberrypi.org/magpi/
Also included is an alternative version, prime-blinkt.py. This uses Pimoroni's Blinkt! LED array to display a colourful counter as each node in the cluster does its work. The payoff is that updating the display has a significant detrimental effect on the speed of the processing, but hey, it's pretty.
To use the Blinkt! version, make sure you have the Blinkt! drivers installed. See https://learn.pimoroni.com/tutorial/sandyj/getting-started-with-blinkt for instructions. Otherwise, use identically to prime.py.