This is one of those things that you really need do correctly and it could prove to be more than just the belt. You may find you have a bad tensioner.

Search this forum on Belt Tensioner M103 for more info.

Whatever you do , do not try to adjust the belt tension with the nut that
sits at an angle at the top near the water pump and power steering pump, until you find and loosen the 19mm lock nut. It's hard to see or find until you know where to look.

Steve