Use a breaker bar to pull the tensioner arm back and you should get more than enough slack to remove the belt and get the new one in place. If all goes well a new serpentine belt is less than an hour of work, 15 minutes if nothing's in the way of the belt path. You shouldn't need to remove any idler pulleys unless they are worn, and the same goes for the tensioner.

which pulley are you removing?

the belt tensioner does not have to be removed unless it is being replaced.

there is a square hole for a ratchet to release the tension on the tensioner

I would follow the instructions in the repair manual so I wouldn't screw something up. It's $30 well invested. The library may even have one on hand.

usually the serpentine built can be put on with out moving anything

on your exact car i do not know

2000 cadillac deville..what tools do I use to remove the pulley and the tensioner to change the belt? Can I do it myself or have a mechanic to do it for me? Estimate on time as well thanks