How do I add support for a custom carrier board for the nano?

I am trying to make and install a device tree for a custom carrier board for the Nano. I have been following the JETSON NANO PLATFORM ADAPTATION AND BRING-UP GUIDE, DA_09361-002, July 2, 2019. At the end of the Porting U-Boot section, I don’t know what comes next after Step 13, the last step. The next section Porting the Linux Kernel does not seem to relate to the previous section. In the Porting the Linux Kernel section Step 1. refers to a .dtb file that does not exist in the SDK. What file should be used? Is this the best document to follow or is there a better procedure?