Serpentine belt replacement please?

I have an easier suggestion. If it's the compressor that locked up, just unplug the clutch coil or remove the relay from the under-hood fuse box. The compressor is supposed to run in the defrost setting, so unplugging the clutch will let the pulley spin freely without engaging the locked-up compressor.

If the clutch is locked up, it will just run the compressor all the time. It sounds like that is not the case here, but if it is, look at the belt routing diagram on the sticker under the hood. It is highly unlikely your car model was ever available without air conditioning, but if it was, that diagram will show both routings and whether an additional idler pulley was used in that application. If the car was available without AC, any auto parts store will have a listing for the correct belt, but you'll have to look at their diagram to see if you have to remove the compressor and / or add other pulleys.

Assuming you have the correct length belt then put the belt on all the pulleys except the water pump. With the tensioner in the loose belt position then slide the belt over the water pulley. This way you don't have to clear the flanges on the alternator pulley

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The routing of the belt can be tricky some times. Usually there is a diagram with the routing. It can be as simple as what point you go around the tensioner. Check your routing versus the diagram and see if it is correct.
If it is, then suspect the tensioner has gone bad. My money is on the routing.