i installed windows on the internal hd running it from a micro sd reduces performance but you are going to need one for the windows install .
use windows 10 , 11 has extra security features that might cause problems .
downloads you need .
windows media creation tool from microsoft website , rufus , steamdeck windows resources from steam website ,
run the creation tool this will make an iso image for you and save it to the desktop , then run rufus to make the iso bootable just remember to select windows to go in the options in rufus and save to the micro sd card , this is all done on a pc , once done insert the card into the steamdeck , hold down the volume button while turning on the steamdeck it will bring up a boot menu select the sc card and windows will install , when windows boots up for the first time it will be in portrait mode but the touchscreen works so go into settings and set it to landscape , install the files you got from steam website and remember to right click the audio files and install from the dropdown menu they are not a normal install file , windows should be installed on your deck . it is basic info on how to do it but if you know your way on installing windows and programs it should be no problem i hope it helps yourself and others .