Tool - RPi Imager

Prerequisite

Download the bootloader image file for your board from kOS Developer’s Portal
- In Mac OS, the file is automatically unarchived, and the kos-installer.img file will be present in the download location.
- In Windows, the user must unarchive the downloaded file and use kos-installer. img for flashing.
Download and Install Raspberry Pi Imager from here
Access to USB thumb drive or SD card or Raspberry Pi board you are flashing should be in USB boot mode.

Run the Raspberry Pi Imager program.

Figure 1.
Click the "CHOOSE OS" button.
From the "Operating System" dialog, scroll to the bottom and select "Use custom".

Figure 2.
Navigate to and then select the disk.img.gz file previously downloaded.
Click the "CHOOSE STORAGE" button.
Select the drive that represents the Raspberry Pi device or storage device that you are using.

Figure 3.
When ready, click the "WRITE" button.

Figure 4.
Click "YES" at the confirmation popup. Writing begins and progress is displayed with a status bar:

Figure 5.
The disk image is written and then verified, which takes a few minutes.

Figure 6.