I have always use a butane lighter to shrink heat-shrink tube onto wire. I use a medium flame and sort of gently wave/brush the flame along the tube to gradually heat it, not leaving the flame in one place for more than it takes to shrink it, rotating the wire and warming opposite sides as I go. It takes a little practice, but I have always found it to be cheap and effective. I keep a butane lighter in my tool box for just this reason. I find that a heat gun will heat it too fast. I just wave the heat-shrink tube through the heat gun blast to slowly and gently heat it if I need to use a gun. A butane lighter gives more control for small sections of heat-shrink, especially if you have temperature sensitive components or housing close to where you are shrinking. If I am doing a really long run of heat-shrink, I pass the wire quickly through the heat-gun blast or wave the heat-gun at it (like drying hair). You definitely don't need a heat-gun (the type used to strip paint) unless you are doing a LOT of heat shrinking and can place the gun on a stand to pass the shrink through the blast.