I have seen it done both ways.
put it on the inside. It is easier and less noticeable from the outside. Plus the air between the window and the inside plastic is more and provides for a better barrier from the cold.
It would depend on which way is easier to access and which will give you a better seal. one benefit to outside is that you can often extend the plastic on past the window panes easier to be sure you seal air not only from the glass but also from any gaps in woodwork. Either way will be beneficial. I would look at where you will get the best seal and where you will do the least damage to trim etc when you staple it, tape it, or however you choose to adhere it
Inside. You don't want moisture to get in as well as the cold. Make sure their is no gaps. Best to do it on a windy day so you can feel for drafts easier.
Inside. Try reading the instructions
depends on your region because you do not want moisture build up in your house but the outside of the window would keep the wind from hitting the window somewhat and does not conduct temperatures like single pane glass. but it probably would not look are last as long on the outside
