For this task, you'll need the 30mm length of heat shrink tubing, the connector you clipped from one of your 25mm cooling fans and the 40mm Rambo cooling fan, as shown.

Cut the 30mm heat shrink in half and set it aside. Strip about 6mm from each of the fan and connector wires.

Slip the heat shrink on to the fan leads and then solder the connector to the fan leads.

Cover the solder joint with the heat shrink tubing and shrink it!