Hardware

Items

Raspberry Pi setup with wifi access
Chassis with 2 indepedantly controllable motors
MotorHAT by UKonline2000 (http://ukonline2000.com/?p=775)
7.2V battery pack to power the motors
5V battery pack to power the pi
Jumper wires
Zip ties to hold things in place, and mounting screws for the pi

Connections

MotorHAT plugged into RaspberryPi GPIO
7.2v battery pack connected to MotorHAT power terminals
Motors wires to MotorHAT terminals (you'll need to test the software to see if it is the correct polarity, but if it is incorrect this only means the direction will be reverse)
HC-SR04 ultrasonic sensor connected to Pi as follows:
SENSOR > PI GPIO
TRIG > 23
ECHO > 24**
VCC > 5 volts
GND > GND

** Connected via a voltage divider to bring the input to the Pi down to 3.3v. This is because the Pi GPIO's work at 3.3v but the HC-SR04 is 5v. See this tutorial for full explanation

Software

SSH into your Pi (I'm starting from Raspbian Jessie Lite), and do the following to setup the Pi.:

sudo apt-get udpate
sudo apt-get install git
sudo apt-get install python-smbus i2c-tools python-dev 
sudo raspi-config
# You will enter into the Pi Config menu
# Select option 9 Advanced config, then option A7 Enable I2C
# then answer yes to enable and enable on each boot. Finally reboot the Pi

Finally, clone the repo. Then, just run the robot.py file and off you go! Press CTRL-C to stop.

sudo git clone https://github.com/andy-pi/berrybot.git
cd berrybot
sudo python robot.py

Python code walkthrough

See the details here: http://andypi.co.uk/2016/05/08/berry-bot/