I just did this at 94,991 miles, and followed the advice to replace the belt and
the idler on the tensioner, oiled the tensioner. It is fairly obvious what to do
once you get started but this video just confirmed it all:

This thread has lots of good advice:

And this one: https://www.cadillacforums.com/forum...last-week.html

I took the tensioner completely out to change the idler and it makes it easier
to get the belt in place.

Used a chop stick to push the belt into place, then once you get the tensioner
back in it seems impossible to see the belt on the water pump pulley. With a
light and looking between the gaps you can check to see that the belt is in the
proper position on the pulley.

Bought the belt and idler from Rock Auto.

This is actually fairly easy, should take about an hour or less if you don't drop
any bolts or tools.