A DIY project called MailPig involves building an IoT mailbox sensor that uses a Twilio SIM and M2M commands to notify users when their mail arrives. The device consists of an Adafruit 32u4 Feather FONA, a photoresistor, and a battery-powered cellular connection, which detects light and sends a text message when triggered. To assemble the device, one needs to solder wires, connect components, and add code using the Arduino IDE and Twilio's M2M commands. The final product is then encased in a plastic Easter egg and attached to a mailbox, where it can be powered by a battery and connected to a cellular network to send notifications.