[Transformer TF300T Q&A, Help & Troubleshooting] martyr2k6:
Alright brother, so here is what I have got figured out since I last posted about, oh, say 8 hours ago...

Now, assuming you took my advice and did the whole lovely NVFlash Tutorial, then what I would do is go to Asus.com, pick your region, go to support, firmware, then select the 10.4.2.20 update that matches your tab's region (mine is in the US, so I picked US). Then download that sucker, go smoke a cig or two (however long it takes) or do whatever you do that will take THAT time to download the ~500mb file...

FROM THERE, assuming you are using windows (like I am, Windows 7 to be exact) use your favorite unzipping prog. to open the file, then extract the .zip within (called US_epad-user-10.4.2.20.zip) to somewhere on your interal (or I prefer external) sd card.

NOW, if you haven't done so already, you need a Custom Recovery, and I HIGHLY recommend TWRP because in my experience, it is simply more versatile that CWM (clockword mod). Again, you can find this in the Developement section of this forum...

Once you have both TWRP, and your official update file on an sd card, you are almost home free here... Alls you have to do is boot up into recovery from, say... device turned off... by first holding Volume Down then pressing and holding (at the same time) The power button until device comes on. You will then be met with a small message in the upper left hand screen saying "press Volume up to enter RCK" (or recovery) and TWRP should load from here.

Once it does, then it's easy here, first, for grins, create a backup, simply by hitting the backup button (you'll know it when you see it) that way you have a dyed-in-the-wool backup of your actual stock rom (THIS CAN BE VERY HELPFUL should you need to return or repair the tab, which Asus will no longer cover since it's unlocked, but if you got it from, say Amazon or Best Buy, you should have almost NO problems being able to do that... jejejeje) once backup is complete (should take about 3-5 minutes or so) then hit the Home softkey in the upper right corner, go to "install" then find where you put that update file that I had you extract earlier (hopefully on your external sd card for simplicity)... You are given a some-what easy to navigate file system that should help you locate it where your folder tree will be on the left window and your files within the active folder are in the right, select the file, then install...

WHEEEEE! Now let that bad boy go, when it says Success! and displays the buttons again at the bottom, then you are good to return home to the main screen (of TWRP) or to just Reboot the tab. Upon rebooting, you will first see your Asus splash screen with the device unlocked text in the upper left, following some new text saying something silly like official update ok, then the next screen will have a slightly different Asus splash with a progress bar that fills up blue as it updates/installs the system. THIS WILL TAKE A LITTLE WHILE, and when done it will reboot again ( I believe) and then show the Asus logo with a dotted rotating "loading" circle, and this too, will take a while to finish doing what it's doing...

From here, it's just the waiting game....until you are prompted to set it up like you did the first time out of the box.

Congrats! Now you are fully up to date until Asus posts the NEW 4.2 update on the site, which is currently only available in the US via OTA update (which would require you NOT be unlocked, but who cares)

You can find info on THAT update below...

http://www.androidcentral.com/android-42-roll-out-asus-transformer-pad-tf300-begins-today-us

Other than that, once you are where you wanted to be (which was on the official JB 4.1.1 update, things will go smoothly for you, it ran real nice for me, but I don't know if it's just my Wi-Fi setup or what, but I seemed to lose Wi-Fi stability after a month or so... Asus has been known for that I hear, but then again, you might not have that issue...

At any rate, you are now able to safely flash your brains out on this tab... I was on CM 10.1 (which gives you the bleeding edge Android 4.2.2) and it works nicely, but I experienced some seriously noticeable lag on screen rotation when using the nightlies that simply just weren't there on the stock rom. Not only that but the stock rom gives you a cute feature at the bottom of the screen, in the middle of the notification/nav bar... a navigation locker (great for games, especially if you have kids or just hit the nav buttons accidently when using media).

I am currently running the newest AOKP, and it does everything that CM10.1 does (as far as I can tell) and isn't short of any features (again, as far as I can tell after about an hour of toying) that CM10.1 has...

*finally exhales* Hope this has been helpful to you... God, I know it would have been helpful to me back when I did this.