Download your flavor of pi os, personally I'm using Ubuntu 22.04 because ROS2 Humble Hawksbill is supposedly the most stable on it 

https://cdimage.ubuntu.com/releases/jammy/release/

I would use Pi flasher so you can modify the values for login and WiFi

https://www.raspberrypi.com/software/

Then plug in that SD card and flash that Pi

There's many flavours of motor hats but this one is mine or at least the one I found cheapest on Amazon

https://www.sunfounder.com/products/sunfounder-robot-hat-expansion-board-designed-for-raspberry-pi

https://www.amazon.com/SunFounder-Expansion-Raspberry-Inclulded-Rechargeable/dp/B0CKTD7QJB

I would not use the bottom mount points and use smaller ones so something like this would work with the M.2 screws that come with the SunFounder Pi Hat

https://www.amazon.com/AVERZELLA-M-2-SSD-Screw-kit/dp/B091B76RRC

For the main body, undercarriage, wheels and horns:

https://www.printables.com/model/964421-tengu-marauder-frame

I ended up using these for my caster and wheels, reccomend you print in TPU for the wheels

https://www.thingiverse.com/thing:4764727

Cover the TPU tire over the wheel and mount it on the TT motor

Take the motors and put on the undercarriage make sure to screw in the motors to the mounts. Use self tapping screws