The best approach to all window drafts is to ensure the window is fully closed. You cannot imagine how many open windows I find. Often the window closure system is not enough to fully close the window. If the window is not closed and the weather is cold outside you will often have ice build-up that prevents the window from fully closing. I recommend waiting for a warm day and using a hair dryer or similar heat source to melt any ice that may be an obstacle. Next get someone outside to push the window closed. If the window will not close don’t force it. It is likely that it is due to be replaced. Finally if the window is still drafty once closed you will want to buy the window shrink wrap to reduce the drafts for this winter and try to insulate the window better in the summer. Remember double paned sealed units are best but if your budget doesn’t allow for replacing the windows you need to make them as efficient as possible using other methods.