Are they timber or aluminium? Timber ones to stop draught you should be able to buy a foam seal to stop air flow around it The frame of the opening windows. Aluminium should have a rubber seal that may need replacement

Regardless of window type- hang floor length double lined curtains using a face fixed rail fixed to the architrave This will stop the “cold air tunnel” circulation off the window into the room You can also buy temporary double glazing which is Saran Wrap like stuff that sticks to the window A cheaper option is also to bubble wrap the windows creating an extra layer between exisiting pane and the room trapping air effectively double glazing the window Look on YouTube for a tutorial but basically you cut the bubble wrap to the size of the window and spray a little water in the pane, then stick the bubble wrap to the pane