Jetson Nano SOM bring up for Custom carrier board

Hi Jerry,
I was trying to adapt Nvidia Jetson Nano eMMC SOM in my customise board.
For that I refer online Jetson Nano adaptation and bring up guide.
I have generated dtsi files and I replace default files contains with generated dtsi file.
Now I want to do porting U boot step.
I guide following steps are given

Porting U-Boot

Perform the following actions in the U-Boot source code to add support for your board.

1.Copy this file to include/configs/.h:

•For original Jetson Nano, include/configs/p3450-0000.h

•For Jetson Nano 2GB, include/configs/p3541-0000.h

2.Edit the set of enabled devices and features in .h as appropriate for your board. For example, define the manifest constant CONFIG_TEGRA_BOARD_STRING:

•For original Jetson Nano, “NVIDIA P3450-0000”

•For Jetson Nano 2GB, “NVIDIA P3541-0000”

3.Copy this file to arch/arm/dts/tegra210-.dts:

•For original Jetson Nano, arch/arm/dts/tegra210-p3450-porg.dts

•For Jetson Nano 2GB, arch/arm/dts/tegra210-p3541-0000.dts

4.Edit the set of enabled devices and their parameters (e.g. GPIO and IRQ IDs) in tegra210-.dts as appropriate for your board.

You may have to add, remove, or edit nodes and properties.

5.Add tegra210-.dtb to arch/arm/dts/Makefile.

6.Copy this file to configs/_defconfig:

•For original Jetson Nano, configs/p3450-0000_defconfig

•For Jetson Nano 2GB, configs/p3541-0000_defconfig

7.Edit _defconfig to refer to your board name.

8.Edit arch/arm/mach-tegra/tegra210/Kconfig to add target config and Kconfig.

9.Copy this directory and its contents to board///:

•For original Jetson Nano, board/nvidia/p3450-porg/

•For Jetson Nano 2GB, board/nvidia/p3541-0000/

10.Edit all of the files in board/// to refer to your board name rather than to p3450-porg or p3541-0000.

11.Edit board///MAINTAINERS to provide the correct maintainer contact information for your board.

12.Edit board///.c to provide the correct PMIC programming for your board, if required.

But I am slightly confuse with the replacing files. I cant able to make out that from where I should
take .h file and where I have to put it.
Can you please guide me on above steps?
So I can able to replace those files.
Also I want to know that when we suppose to run
$ make ARCH=arm64 tegra_defconfig

$ make ARCH=arm64 dtbs
commands before porting Uboot or after porting the Uboot?