You would install your floating floor then install the transition pieces after that. You’re gonna need the transition pieces first to determine the final termination point of the flooring. From there you will work backwards and subtract the thickness of the reducer plus any expansion space that is required. The combination of those two measurement will tell you where your floor needs to end. You would then install the flooring up to that point and your reducer, plus any expansion space, would carry the installation up to the final stopping point.