Don't disconnect until you can rewrite that to say I have successfully air layered . . .
Seriously can you see roots in the sphagnum? Unless you have to do it because of the imminent advent of freezing weather, don't disconnect the layer unless you can see a LOT of roots. When you disconnect it, plant it -- sphagnum and all -- into a large shallow pot of bonsai soil. TIE it into the pot. Keep it outside as long as possible, but you shouldn't let the roots freeze hard this year, so bring it inside for cold nights, but do not keep it inside. When the days are going to be above freezing it will appreciate being outside. Move it carefully so you don't jostle it too much. The new roots will be tended. In late spring, repot, gently washing off the sphagnum.