The Raspberry Pi is already a fairly tiny piece of hardware, but with some tinkering, it's possible to go even smaller. Modder NODE, who's hacked a few Pis in his time, has a new project doing the rounds -- a fully-functional Raspberry Pi 3 that's a mere 7mm thick.
As the video above explains, the trick is to remove all the bulky components on the original Pi 3 board -- so all the external ports, essentially. This includes the ethernet and USB connectors, which for this mod were "sawn off":
Inside there is a modified Raspberry Pi 3, with all the major ports and components removed. The end of the board which housed the USB and Ethernet ports has been sawn off, and the three micro USB breakout boards have been soldered directly to the main board.
A custom 3D-printed case holds everything in place, the major feature a top-facing grill to keep the "notoriously warm Pi 3" cool.
While NODE doesn't provide explicit instructions, the project page has everything else you need. Considering the mod requires mutilating the board, it's definitely not for the faint of heart.