Step 1: Things you need
First you need a raspberry pi, no matter if you are using a model A or B. I used a raspberry pi model B because it has got more RAM. Then you need a screen for it, I used a 4.3" TFT car monitor. The screen should accept 5 v, for 3.5" or 4.3" car monitors are lots of guides how to make them working with 5 volts. And you need a USB battery pack, it should have at least an 1 A usb output. You will also need a micro usb AC adapter if you have no. For the buttons I used a snes usb controller. You even need an ON/OFF switch and some wires. Last but not least you need some cables, one with an usb female port, another one with a female micro usb connector, one with a female 3.5 mm headphone jack (I used some extension cord for usb, micro usb and 3.5 mm headphone) and finally 2 USB to micro usb cables.
You should also have some tools and solder skills.