The dd step is to overwrite all the data; this would do the same thing on a HDD or an SSD, but the blkdiscard -s tells the SSD to erase all blocks. In theory this is a secure erase, but in practice most SSD’s implement it in a flawed way, hence the first step.

Boot up a copy of your favorite Linux Distro, open a terminal, and run lsblk From that, find your deck’s SSD - Probably somthing along the lines of /dev/nvme0n1

Then do sudo dd if=/dev/urandom of=/dev/nvme0n1 bs=4M status=progress

Once that’s done, do sudo blkdiscard /dev/nvme0n1

Your Steamdeck is now very securely erased.

Boot up the recovery media from Valve - https://help.steampowered.com/en/faqs/view/1b71-edf2-eb6d-2bb3

And use the option Re-image Steam Deck from the Desktop. That will set it up as if it’s new from the Factory.