Three examples of joining outer and inner tubes
Top: A straight cut, inner tube going all through the outer.
Middle: Slanted cut, inner tube just stuck into outer.
Bottom: With rear weight. Outer tube shortened to fit.
You tie the hackle/wing section on the part marked in green, right in front of the outer tube or weight. In the middle case the thread will secure the two tubes. In the two other cases the collar melted in the rear and front will keep things in place.