Slacken the large bolt (17mm) that goes through the centre of the tensioner. Then you loosen the adjuster nut (13mm) and the pulley moves back so you can fit the belt. Set the adjustable plastic arrow at the end of the cast triangle and tighten until it's at the other end. Finally tighten the 17mm bolt.
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)