From the RA menu (small left select button and Y on my deck) you can now configure your controls and all the good stuff that RA offers you. Go to Retroarch (standalone) then load the core as Beetle and select saturn Rom. Install Retroarch flat pack if you haven’t done so already then exit to gaming mode. Don’t bother with input controls at the moment as you can do that in RA later. Select the shaders and select ‘goat’ for scanlines if you like. Sound options should be put to ‘DL’ or something like that in the drop down. Point the path to your bios folder once you find the firmware drop down. It’s in ‘paths’ I believe in the global setting but I can’t remember. You first need to go to an option though where you can see a firmware drop down. Set these by going to global settings first then secondly select your system and tweak those settings if need be. While you're still in desktop mode open up Mednafen. go to your file explorer and put your bios x2 (you only need the two really) in the emulation/bios/Saturn or saturnjp folder and your roms in the rom/saturn or saturnjp folder depending on what region they are like you normally would. Go to the discover part of your deck in desktop mode and download Mednafen. It’s better than anything else I’ve been able to find though lol.
So here we are but this is only from memory mind so may not be 100% correct. I had a nightmare setting up SS on the deck and there are zero guides on Youtube or Google to assist.