I've not done it...but looking at manual there is an adjustable untoothed idler tensioner....
Its a one peice timing belt cover....so I'd guess all the belts, fan and external pulleys need to come off to allow its removal.
Book says every 60,000 miles
I think the 3L engine repair manual was online at a hilux/surf site somewhere
I'd be inclined to put a new water pump on at the same time.....
The engine manual is 'toyota rm123e'
Quick search shows some ' torrents' ..but I'm not sure how to use them