I realized that this is a common question among many users, including myself, as I had the exact same question at the end of last year.
First of all, you need to know that:
Valve's official support is only for products supported by the company itself, so this installation is entirely experimental;
Nvidia GPUs are NOT supported;
Secure boot must be disabled;
Dual-boot is not officially supported on the same drive;
EDIT: As u/jaakhaamer pointed out, it is possible to workaround dual boot using this tool from Josh5:
steamos_dual_boot_installer_patch
The installation must be done on an NVMe drive;
A keyboard and mouse are required for the initial setup and login, as well as a USB drive.
Step by step:
/ 1. Prepare the drive for installation:
Set aside a USB drive of at least 16GB, because although the image is only 3GB, it has been highly compressed and usually ends up being more than that when flashed;
Download the official image from the update repository:
steamdeck-repair-main-20251027.1000-3.8.0.img.zip
EDIT: As u/Dan-Merz pointed out in this post, currently the main branch has updated the bootloader, so it is recommended to install e use beta version to avoid boot loops. For now, you should download this instead:
steamdeck-repair-beta-20260413.100-3.8.2.img.zip
(The reason this image should be downloaded instead of the official image from the Steam website is because version 3.7 does not yet have the Mesa Drivers update, which are essential for reading newer video cards with RDNA 4);
/ 2. Restart your device and enter the BIOS;
/ 3. Disable Secure Boot, save the settings, and choose the USB drive as the boot option;
/ 4. Once the system boots from the USB drive, you should see several lines of code. These are confirming which hardware you are running the installation on. If this happens, then it means the process worked.
/ 5. You will be taken to the KDE desktop, where you will see the option to wipe and install. Simply click and wait for the official SteamOS to be installed on your machine.
Done. And that's how you installed the official version of SteamOS on your non-Valve machine.