How do we transition from co-sleeping to sleeping alone?

Don't do it all at once; proceed in stages. 1) Same futon → adjacent futon. 2) Parent lies beside but doesn't touch. 3) Sit and watch over them. 4) Only calling out. 5) Leave the room. Extend the distance one week at a time. If they cry partway through, it's fine to go back one stage. Moving without rushing, going back and forth, actually gets you there sooner in the end.