I followed the serpinetine belt routing diagram for 1994 lincon mark VIII that was on the hood and double checked to make sure it was the right belt and the belt is coming forward off the pulleys.

THE 4.6 DRIVE BELT GOES AROUND AC COMPRESSOR,CRANKSHAFT PULLEY,ALTERNATOR,AND POWER STEERING PUMP, TENSIONER PULLEY SHOULD BE ON THE OUTSIDE AND IDLER PULLEY OUTSIDE DRIVE BELT.WHERE CRANKPULLEY IS OUTSIDE DRIVE BELT GOES AROUND WATER PUMP IN SIDE BELT GOES AROUND POWER STEERING PUMP PULLEY AND ALTERNATOR AND AC COMPRESSOR. IF DRIVE BELT DONT STAY ON PULLEY MORE LIKELY DRIVE BELT TENSIONER FAULTY.

Then it is too loose. Double check but I'm pretty sure your adjustment is on the Alternator bracket. You loosen the bottom bolt which allows it to picot on the bracket, then loosen the upper bracket bolt and pull it outward to tighten it, lock the bolts back down and it should resolve your problem. If you have a spring loaded assembly that applies tension and you have to release to get the belt on and off then you may need to replace that or your not getting the belt all the way on. If even one of those channels on that belt don't line up properly the belt will work itself off. If you have a frozen or damaged pulley it will do this as well.