Having SSH and SFTP access to your steam deck is very convenient for file transfers and remote administration, setting it up is fairly straightforward.
First you need to have set your password to be able to use sudo, to do this open konsole in desktop mode and type passwd, enter your desired password twice to confirm.
Once that is done you can turn on the SSH server with sudo systemctl start sshd, to login from another system on your network use ssh deck@192.168.x.x, replace with the LAN address of your deck.
Security
To prevent anyone else logging in, edit /etc/ssh/sshd_config, add the following two lines:
AllowUsers deck@192.168.0.0/16
PermitRootLogin noRestart the server with sudo systemctl restart sshd, this will only allow a login from your local network, if you want to login from the internet then more stringent security is required which is covered in an article here.
To permanently enable the server use sudo systemctl enable sshd.