Belt installation instructions needed

Hello, sorry to hear about your issue. What exactly came off? Did you mean a belt came off or something else?

https://www.2carpros.com/articles/how-a-serpentine-belt-works

https://www.2carpros.com/articles/replace-serpentine-belt
Includes Videos

By this diagram can you tell me which one came off? I only see 2. The smaller one is for the power steering pump.
I would possibly suggest finding a new mechanic if a belt came off only 10 days after a repair, that absolutely should not happen, especially since it's probably your power steering belt.

Okay, I see, the instructions are in Red below. In Green is the belt Routing. Make sure the belt is not broken. Start with the Idler Pulley, loosen the nut in the middle of the Idler Pulley, but don't take it off. Just enough so the Pulley can slide to be adjusted by the Upper Bolt.

Next Loosen the Adjuster Bolt in the direction of the Red Arrow. You'll notice the Idler Pulley starts moving upwards. Adjust it only far enough to get the belt on.
With the Belt, Start with the Air Conditioner Compressor, Then to the Crankshaft Pulley, Next the Alternator. and finally slide it on the Idler Pulley.
Make sure that it is lined up in the Groves on all the Pulleys, except the Idler Pulley because that doesn't have any Groves.

Once the belt is in place, start to turn the Idler Pulley Adjuster Bolt (up top) the opposite way to tighten it.
You'll notice the Idler Pulley moving downwards and tightening the Belt.

This is the important part. only tighten the belt so much and check it.
As a rule of thumb, when it adjusted you should be able to twist the belt 90degrees with your fingers. At a point between the Crankshaft Pulley and the Air Conditioner Compressor at the bottom, (this is where the belt has the longest point where it's not touching any other pulleys). It should be able to twist a maximum of about a 1/4 the way around by hand. Hopefully you understand what I'm saying. I'll put it on the diagram.

Once you get it tight enough, you'll need to tighten the Idler Pulley Nut that you loosen in the first step. It should only take 2 wrenches to do this.
It may look intimidating, but the most important part is getting the belt tightened enough and tightening the Idler Pulley Nut, (which it seems is what your mechanic forgot) or there's a problem with one of those other pulleys.
If the Alternator, Idler Pulley, and Air Conditioner Pulleys don't move by hand, then something seized up and you will have to have it towed to the mechanic.
If you have any questions just ask and take a picture of it when you're done so we can see it.
There's a video in this guide below, but your car is different when it comes to having to tighten the belt. It might show you how to line up the Groves, but really the belt will fit right into the Groves, you should be able to notice that...
https://www.2carpros.com/articles/replace-serpentine-belt

If you have any questions at any point, I'll be here all day

https://www.youtube.com/watch?v=QR6iRch-FAc
Here is another video to help, He removes the tire to do this, but you can probably do it without removing the tire. You'll need to reach down and feel that the belt is on the Crankshaft Pulley in the grooves..

Well, he may not have noticed it, there is usually a rubber ring that's built into the harmonic balancer and that's the part that comes apart, the inner section stays attached to the crankshaft and the outer ring comes off. But $400.00 seems to be a lot to just replace the balancer. Its only one bolt in the middle and then we use a small puller to pull it off straight. I'm not sure why he's charging you so much. It's only a $50.00 part.
This picture is from AutoZone.