Use the heating gun to to dispense heat evenly at the base of the shaft where it enters the club head and the portion of the shaft seen at the sole. This will melt all the old epoxy and allow you to remove the old shaft without harming it.
Remove the old shaft and keep it so you can use the tip as a guide to cutting the new shaft so it will fit properly. Allow the club head to cool completely and use a small piece of rolled sandpaper to clean the newly-exposed hole.
Cut the tip of the new shaft to match the tip of the old shaft, using either a knife or a razor blade, depending on the type of shaft.
Drip epoxy evenly into the exposed hole. This can get messy, so it is best to do this in a garage or over a throw-away protective covering.
Fit the replacement ferrule onto the tip of the new shaft so that it will fit flush against the top of the club head once the shaft is inserted.
Fit the head onto the shaft so that the angle conforms to sole of the club head and the small ferrule ring is snug against the top. Always slide the head onto the shaft and never the other way around. It is much easier to manage and negotiate pressure if the shaft is stationary.
Allow the club to set for at least 24 hours before using.