Not very easily unless you tell us the exact make and model of your TV set (if it's got built-in Freeview) or of your set-top Freeview box (if that's what you're using). They're all different!
However simply looking for an 'AD' button on your remote, or pressing the 'Menu' button and scrolling through the options to look for an 'Audio Description' option might be a good place to start.
thanks Chico - built in Freeview - and it is a Samsung. When you look at the description of the programme - after it says AD/S - which as you know means audio description and subtitles.
As has been said, look for a relevant button on your remote control.
However, taking a look at the manual for a random Samsung model, you might need to press 'System', then go to 'Accessibility' and lastly 'Audio Description' to toggle it on and off. (I'd need the exact model number, such as 'Series 5 H5510' or 'UE40H5510AK', from the back of the set or from the box it came in, to be sure at looking at the correct manual though).