With pre-tubed, you just soak ahead of time. You don't have to flush the inside out like standard casings. What I did at first was pack the leftovers back in salt, vac seal, and keep in the fridge.The problem is, all of the advice wants them soaked for some time and to have water run through the inside of the casing.
I personally found the pre-tubed more of a hassle to work with, so I pulled them all off the tubes and put them in a jar of brine.