Let me start this by saying that I have learned a lot from this forum and found its members to be very helpful (when asked the correct way). I decided to do this write-up for the non-mechanically inclined, “I have never worked on my car before” 4runner owner, which I was before today. Yes I used a kit, yes I could have saved money doing it with the parts that are found on some of the mod links I’ve listed below, but being a modification virgin I wanted to make sure it was done right, with the right pieces the first time.

Background:
I have never worked on a car before. I am handy with tools, and do build things, but have not done much more then change flat tires before on my cars. I did put my 1987 Ford Escort’s air intake manifold back together with just duct tape when I was stranded in College Park after a football game and it got me home (but that’s another story). As many of you know being a part of this forum starts to fuel an addiction and I was starting to get the itch to make the vehicle “mine”. So after reading the posts about the rear differential when submerged and experiencing more flooded streets then I’ve ever seen now living in South Florida I figured this is one that I could try. After doing the research I know I most likely don’t need this mod, but I’d rather be safe then sorry, and most trails in Florida seem to have water in/around them in some fashion. I am the type of person who would try and avoid water crossings if possible, but still I want to be and the vehicle to be prepared if there are no other options.

Acknowledgements:

I could not have done this mod without the wonderful write-ups and information provided by @Canazes9 , @Okki, @Saker , @Boyds4Runner , @DesertCanyons , and @MTN4RNR on the threads below along with their build threads. (There were also many other helpful contributions, too many to continue listing here)

Previous Threads & Links Used To Complete Mod:
“Rear Differential Complete”
Rear Diff Breather Mod - Complete!

“Breather Mod- All Tied Into One”
Breather Mod - All tied into one

You Tube ARB Install (edit: when the installer is talking torque specs he is means 30in/lbs of torque not 30ft/lbs).
https://www.youtube.com/watch?v=v5FDbks-R1g

Equipment Used:
ARB Breather Install Kit
Phillips Screwdriver
14 mm or adjustable wrench
Cordless Drill (if mounting with self-tapping screws included in kit)
Pocket Knife/Utility Knife

Time:
1 – 2 Hours (Depends on if you have a 3.5 year old helper or not)

Without further adieu, the Breather Mod for Dummies!

I used the ARB Differential Breather Kit for this modification. I bought it from Amazon.
Amazon.com: ARB 170112 Differential Axle Breather: Automotive

The kit includes a filter, 4 two-way breather fittings with Teflon tape already on the threads, 3 plugs for any unused breather fittings in the filter, lots of zip ties and enough tube to do multiple breather modifications.

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Step 1:
Remove spare tire to locate the OEM one-way rear differential breather. It looks like a sliver-capped plug.

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Step 2:
Run the breather line (but do not zip tie or attach anything yet). I started in the engine bay, and ran it to the back of the vehicle. A lot of people run it up to the gas tank or up the fuel tube, but I wanted something up, out of the way and easily accessible if needed. I taped the end of the tube I was running down through the engine and along the different lines and hoses to the back of the vehicle so that I would not get dust/dirt in it.

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Once I fed it down the engine bay I worked it behind the fender liners to keep it away from the shocks and front axle. I then found some lines to run it next to above the heat shields and above the driver’s side frame rail.

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I then followed those lines above the gas tank/gas tank skid plate. Making sure I could eventually zip tie to other lines that were above the center differential.

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I'm looking forward to reading your post. I take it you're still writing it.
Cudos to you for completing it in 2 to 3 hours with your little helper. Mine took me 7 to 8 hours total as I recall. Here's the link to my Diff & E-Locker breather mod:
E-Locker and Differential Breather install, Trail Edition
I had to build mine all by myself. My grandkids live too far away. Wish they lived closer...

I continued to follow the lines (I believe they are the fuel and electrical) above the rear axle and differential.

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Step 3:
Replace OEM Breather with ARB two-way fitting. Use the wrench to unscrew the old breather. There will be a gas release as you finish unscrewing from the vacuum within. Install the ARB until completely hand tightened.

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Step 4:
Insert Breather Line into ARB fitting. Before I did this I made sure that there was an extra coil of slack before inserting the line into the ARB Breather. This allows for suspension travel (I still have not used any zip ties yet). The line pushes directly into the fitting and seals itself. Once inserted it is locked in (YOU CAN NOT PULL THE LINE BACK OUT AT THIS POINT SO BE SURE YOU”RE READY TO INSERT).

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Step 5.
Zip Tie: I now went back through and zip-tied the Breather Line to the electrical and fuel lines that I chose to run along. I made sure to leave some slack near the breather for suspension travel and that the zip ties were not over tightened so the line was attached but could still move freely.

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Step 6
Install filter bracket. I decided to install it on the upper driver’s side using the self-taping screws. This gets it up and out of the way, if water gets in here I have other major issues, not just my rear differential. Since I only am attaching one breather at this time I used the caps in the other 3 breather holes on the filter. I then attached the one other two-way breather fitting and attached it using the self-taping screws and a cordless drill.

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Step 7
Install the breather line into the filter bracket. I made sure that there was slack, giving it another loop up and down and then cut off the excess breather line. This is in case I decide to do a lift in the future, or some other modification where I need to extend the breather line. I then inserted the breather line into the two-way ARB breather fitting, pushing it in and locking it in place (same way as on the rear differential fitting). I then loosely zip tied the line to a couple of the electrical lines.

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Step 8
Screw on Filter. (Self Explanatory)

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Step 9:

Finish with one of your favorite adult beverages. I picked Occulto from the recommendation of @1engineer . If you are having difficulty check out other recommendations from page 2 on of this thread.
My First 4WD Experience with my '16 T4RTE - NOT GOOD