Thought I'd write a guide:
(it will only work with this FW: https://www.overclock.net/t/1561041/reverse-engineering-3366/300_100#post_26314321
If you just want to use the FW as is, you just need to connect the pins as so:
3360 NCS (SS): PB0
3360 SCLK (SC): PB1
3360 MOSI (MO): PB2
3360 MISO (MI): PB3
3360 NRESET (RS): No need to connect
Wheel B: PC6 (wheel up)
Wheel A: PC7 (wheel down)
Left button: PD0
Right button: PD1
Wheel button: PD2
Back side button: PD3
Forwards side button: PD4
DPI button: PD5
Don't forget the ground and power connections. There are 2 VCC and 2 GND on the teensy, doesn't matter which ones you use. You will need VCC and GND to go to the 3360, and GND to go to the ground plane for the switches (ie. connect GND to a ground pin on one of the switches, and that pin will be connected to the ground plane). Just be sure the ground plane goes to all the switches and there isn't a gap somewhere, if there is a gap, then you need to use a wire to bridge that gap (which will be a wire going from either GND on the teensy or a switch GND pin that has a GND connection, to the GND pin on the switch that isn't connected to GND).
Connecting the USB plug:
This is the most fiddly part of the whole thing. This is the connector I use: http://www.ebay.co.uk/itm/262814319020
Take a cable from another mouse, or get a paracord.
Heres how I connected it, looking at the back of the connector, from left to right, top to bottom:
Top 1: No connection
Top 2: White, (D-)
Bottom 1: Black shiny (GND) (Don't connect SHIELD to this! Usually it has a matte black cable sleeve, just snip it off as it won't be needed)
Bottom 2: Green (D+)
Bottom 3: Red (5V)
Be very careful and make sure there are no short circuits, I'm pretty sure short circuiting 5V and GND would cause irreparable damage to your mobo.
1) Download the Teensy Loader from here: https://www.pjrc.com/teensy/loader.html
2) Now follow the instructions: https://www.pjrc.com/teensy/loader_win10.html
If you want to edit the FW, then you can just use something like notepad++ to edit the main.c file. To compile it, you should use the AVR GCC toolchain. For this you will have to use Linux because the windows version is too old. Its not really a big deal though.
1) Download Oracle VM Virtualbox from here: https://www.virtualbox.org/
2) Follow instructions here to install Ubuntu: https://askubuntu.com/questions/142549/how-to-install-ubuntu-on-virtualbox
3) Once thats all done, load up Ubuntu and follow these instructions to install the AVR GCC Toolchain: http://maxembedded.com/2015/06/setting-up-avr-gcc-toolchain-on-linux-and-mac-os-x/
4) Now, all you have to do is transfer your code into the shared folder, then, in ubuntu, go to the shared folder (the one that has your main.c etc inside), right click and then click on open terminal in this folder. Once the terminal comes up, just type "make" without the quotes, and it'll compile your code. Then you'll have a hex file which you can use with the teensy loader.
The FW is coded to allow you enable bootloader mode by holding down the DPI button when plugging in the mouse, so you don't have to keep opening the mouse up and pressing the button on the teensy.
Its this bit, so don't remove it if you want to keep that functionality:
// if dpi button is pressed when plugging in, jump to bootloader
// see https://www.pjrc.com/teensy/jump_to_bootloader.html
if (!(PIND & (1<<5)))
__asm__ volatile ("jmp 0x7e00");
Hope that helps!