G - Depends on which insulation you buy determines how to install it. The simple way - if it is sheet metal, insulate it. For each supply and return duct that is exposed to outside temperatures, they affect the temperature inside your house and how hard your system is working=energy burn. If you are in summer and it is 100 degrees outside (like 80 under the house), than your AC unit is working to deal with a duct that is surrounded by 80 degrees. Reverse in the winter. The goal is to insulate it enough to minimize the outside world. So I would insulate every piece of sheet metal you can reach. Same as dual pane windows, insulating walls and ceiling-they all affect outside temps from coming into house. There may even be a rebate from your utility company to do it. I would say a minimum of R-4.2 insulation on the duct, but probably standard should be R-6 and now there is R-8/R-10. If you are going through this trouble it makes sense to reseal all joints before you insulate, just to minimize any air leakage. There may be rebates to seal duct as well. Of course you can rip it all out and install new flex duct that is already insulated and sealed from the factory. I can help you with costs on this stuff as well.
Something like this