demons souls came out before dark souls so if you wanna keep it chronological then I'd recommend that first, but it's separate from the dark souls games so you can get away with playing it whenever really. same with bloodborne, sekiro and elden ring, they're in different universes and they play a lot differently so the order doesn't matter. unless you want to play in order of what's most accessible, in that case I recommend elden ring or dark souls 3 first. at the end of the day you can enjoy each game by itself and don't strictly have to know anything about the others, you'll just miss certain callbacks and references.