The older XJ's use the power steering pump as the tensioner. Loosen the mount bolts on the pump and you should be able to see a threaded rod that will loosen when screwed in or out (it's been awhile!). Once the belt is swapped, do the opposite.
replacement of S belt.
both 4 or 6 cyl should be the same procedure.
4cyl:
Belt tension is adjusted at the power steering pump. Located there are two bolt, one for adjustment and one for pivoting....