To flash any sort of custom ROM like the AOSP Android 4.1 jelly bean ROM for the Galaxy tab 8.9, you need to have some sort of ClockworkMod Recovery (CWM). With the ClockworkMod Recovery, you will also be able to install myriad unofficial updates that keep popping up on XDA and sources like sammobile. Here are how you can download ClockworkMod Recovery to your Samsung Galaxy Tab 8.9 tablet.
1) Download the ClockworkMod_Recovery_v22.214.171.124.tar from here and reboot the tab into download mode.
2) Start Odin 1.85 and put the ClockworkMod_Recovery_v126.96.36.199.tar in the PDA field and click on Flash button.
3) Reboot into ClockworkMod Recovery.
To revert back to stock recovery, you can either flash the stock recovery or flash the factory ROM for the Galaxy tab 8.9.