Roll back the insulation and outer covering of the flex duct to expose the inner liner on both pieces of flex you need to join.
Cut the inner liner of the flex square using the flex knife. Trim the excess wire that projects from the inner liner. Perform the same operation on the inner liner of second piece of flex that you need to join.
Place a center mark at four points of the metal sleeve by measuring 2 inches in from either end of the sleeve with the tape measure and permanent marker. Make sure that the marks are equidistant around the metal sleeve.
Slide the metal sleeve into the inner liner of one of the pieces of flex duct. Stop sliding the sleeve when the inner liner reaches the center marks you placed on the sleeve in the previous step.
Wrap a tie strap around the flex duct inner liner and the metal sleeve. Hand tighten the tie down to hold the inner liner to the metal sleeve.
Align the inner liner with the four center marks located on the metal sleeve.
Tighten the tie band with the banding tool.
Slide the inner liner of the second piece of flex on the metal sleeve. When the two inner sleeves meet, secure the inner liner of the second flex with another tie band and banding tool as described in step seven.
Unroll the insulation and outer covering of both pieces of flex. Ensure that the insulation overlaps.
Tuck the outer covering of one piece of flex under the outer covering of the other piece of flex.
Wrap duct tape around the area where the cut edge of the outside covering rests on the other piece of flex to seal the connection.