I am not a professional and will not be held responsible for any damages or injuries caused by these instructions.
The owner’s manual recommendation is inspection of the drive/serpentine belt every 60K miles.
Due to the amount of time required to access and remove the belt for inspection, I just replaced it. For me, around 100K miles is when I replaced it.
Unlike my other “How to..” threads, I didn’t take pictures of every step, but I hope this tutorial will help you through the process.

Tools and supplies used:
Drive belt. Toyota P/N: 99367-K1550
10mm hex socket
ratchet
Flat head screwdriver
5mm L shape allen wrench
14mm wrench (offset or flex head) or 14mm socket
Torque wrench, capable to tighten to 5.0 N-m or 44 in-lbf

1). Remove the (9) plastic clips holding down the duct seal and air duct simply by pushing down on the center portion of each clip then lift up from their base.
Small flat head or trim removal tool may come in handy to pry from the clips bases if they are tough to remove by hand.
2). Remove the engine cover.
Note: The official names by Toyota of “duct seal” and “air inlet” are slightly different, but for simplicity reasons, I will be referring to duct seal and air inlet from this point forward.


2). Once the duct seal is removed, remove the air inlet.
I didn’t take a picture with the air inlet still attached, but there is (1) bolt that secures the air inlet.
Remove the bolt with a 10mm socket.


3). There is a hole near the bottom of the tensioner to keep the tensioner in place in order to relieve tension from the belt.
Using a 14mm wrench, place it on the tension and turn it counterclockwise.
Once you turn the wrench maybe 1/8 of a turn if that, place the 5mm allen wrench in the hole while continuing to hold the 14mm wrench. Once the allen wrench is in place, slowly remove the 14mm wrench.
Note: For me, I didn’t align the two holes exactly as you can see in the picture, but the tensioner was rotated enough for me to remove the belt in the next step.

4). With the tension relieved, remove the belt from the pulleys. Spacing is tight around the crank and water pump so just take your time.

5). Even with 100K miles, the old belt didn’t have any cracks which was good to see.



Comparison of old vs new


6). After confirming your new belt is the same as the old belt, route the new belt around all the pulleys correctly.
See my unofficial diagram below.
Note: Diagram is for an IS350, but I believe the IS250 belt routing is the same.


7). Confirm belt is placed correctly along the width of every pulley.
I visually checked placement on each pulley and around all portions the belt contacts the pulleys.
The places I couldn’t visually check like the bottom of the crank or water pump pulleys, I placed my finger on the edges of the belt and pulley to ensure there is a small gap on both sides.

8). Place the 14mm wrench back over the tensioner bolt and turn it counterclockwise again in order to remove the 5mm allen wrench from the hole.
With the allen wrench removed, slowly let the tensioner engage the new belt.
Reconfirm belt placement on the pulleys.

9). Start the car and visually inspect if the belt is tracking properly around all the pulleys.

10). Reassemble the engine cover, air inlet, and duct seal.
Note: When tightening the (1) bolt for the air inlet, tighten to 5.0 N-m or 44 in-lbf per the service manual.

 

How long of a job from start to finish?

 

Around 1.5 hrs while taking my time. Took me several minutes to figure out how to remove the plastic clips since I haven't had to remove them before.

 

Thats a good write up sure it will help some folks.

Never did anything like the Allen wrench trick, but never did this car. Is it designed that way?

The only thing I would add is instead of "may want to check"...Definitely check belt is in place once fully tensioned back in place. Thats what counts.

Then leave the hood open and start the car and watch for min to insure its working smoothly

Always good to have the diagram you included before you start. Doesnt look complicated until you go to put them back on lol.

 

The hole for the Allen key was purposely designed for this reason, to lock the tensioner in the release position so you can easily route the new belt.
Obviously it can still be done without this, it just makes the install a tad easier using this method to lock the tensioner.

 

Homer thank you for the suggestion. I corrected the original post and also rearranged "starting the car" before reassembly. Easier to see the belt with the components removed.
Regarding the allen key, I read about it in the service manual. As Sasnuke mentioned, some tensions are designed to lock in place, but due to the hole location on the tension for our vehicle, I'm pretty sure I wouldn't have found it without reading the service manual.
An alternative to an allen key would be a stubby screwdriver

 

Just did this, 1 thing to add, that once you get your belt mostly seated ( i prefer to start from the bottom up ) , take the 5mm wrench out of the hole and turn the 14mm socket more to the left for more slack on the belt and slip the rest of the belt in,
found that the 5mm wrench trick makes the belt really tight, could just be an is250 thing however.

 

Thanks for the how to documentation.