For this Instructable you will need:
A Raspberry Pi with SD card inserted and OS installed (if you want to be pre-emptive with your case a credit card will help you see how it all goes together in place of the Pi),
A Sega Mega Drive/Genesis II (See the second picture if you're unsure which version you have),
A screwdriver that can undo all of the screws on the console (they're all the same size),
Some sticky tape,
A micro USB cable and power adapter for your Pi,
An HDMI cable, the thinner the better (or an RCA connector cable would work too),
(Optional, although highly recommended) A 4 port USB hub which is no wider than 105mm (about 4 inches), no taller than about 9mm (0.35 inches) and ideally no more than 30mm deep (1.2 inches). I used an old Kensington one. If it has a short USB cable attached, like mine, you may also want a USB extender cable.
USB Mouse and Keyboard (or any other connection type that you can get to work),
(Optional) Ethernet Cable.
Step 1: Dismantle your Mega Drive/Genesis
Keep all the screws safe (see picture 2; I unscrewed 9 in total but the number and type may vary slightly by model).
I used this video to help me take apart my Mega Drive as it shows you where to look for all the hard-to-find screws. Ignore the bit after 2:40 where he starts soldering; this is unneeded.
In the end you should end up with (see picture 3):
2 pieces of metal,
2 shells of plastic,
a plastic bar and
a load of screws.