This very accomplished video from Mike Beel shows guides you round the Langstone Harbour Ride shown on the back of our Cycle Map.
The illustrated route uses the shared cycle path from the Langbrook Carvery south along the main road to Mill Lane (in red here). This is narrow in places, and passes several obscured drives.
A quieter alternative (shown in blue) is to cross the main road at the Langbrook Carvery into Woodberry Avenue which becomes Southbrook Road. Turn left into the small cul de sac after Hamilton Close (also called Southbrook Road) and use the tucked-away cut-through on the right at the red dot to join the Langstone section of the Billy Trail and on to rejoin the described route at Mill Lane.