[Galaxy S II Q&A, Help & Troubleshooting] UpInTheAir:
Basic Steps

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1. Follow Chainfire's Thread for easy root for noobs. Just follow the instructions exactly! This seems to the closest CF-Root Kernel match KF4

2. As per instructions, use Odin to flash CF-Root Kernel. This will give you root and a custom recovery called CWM (ClockWorkMod), allowing you to flash ROMs, themes, kerenels that are flashable .zip directly from sdcrard (no need for Odin). You can also make "nandroid" backup in CWM, basically taking a system snaphot so you can restore that if anything goes wrong, or don't like the theme you just flashed etc. It saves to sdcard. I strongly suggest this before flashing any custom ROM. It also installs SuperUser and CWM Manager.

3. As you wish to update your Android Version 2.3.3 to 2.3.5, boot into CWM Recovery (power off: hold vol down+home+menu until recovery screen)and factory reset, format cache and dalvil cache (go to advanced), Home button selects, while vol up/dn.

4. Immediately flash custom ROM (ROMs come with kernel, but you might like to try another later) and or kernel via recovery directly from your sdcard.

5. First boot will take a few minutes.

6. As the ROM will have it's own modem, I suggest you give it a try first (check for DL/UL speed, getting H. H+ etc, signal strength etc), if not, check my signature for DL a CWM flashable specific Telstra Modem DVKF2. This might help.

Recommendations

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1. All personal taste, but, check links in my signature and read each Developers installation guide and FAQ. CheckROM (KJ3) can be highly customizable, if you want that is.

2. It come with kernel & modem. I use siyahkernel v2.2 kernel with governor set to "smartassV2" works wonders for battery life. There are many apps to control the kernel governor and CPU frequency, including SetCPU (search XDA - free for members, and donate version in market if you really like it). Be aware that this kernel will update to a newer CWM v5.xx, and any nan-backup made with prevoius CWM v4.xx will not restore.

3. Also download Hellcats tool from the Market to backup your EFS folder. This is where your IMEI is stored, and you don't want that to ever get corrupted without having a good backup. Saves to sdcard. Do this before flashing any ROM first time.

4. Flash recommeded official Telstra Modem, link in signature from sdcard via CWM Recovery.

5. Samsung USB jig - check edit1......

Edit1: This method of rooting is by far the easiest, but will give a warning triangle on boot and increase the binary flash counter shown in Download Mode. There are other methods, but the above "problem" is easily solved by purchasing a "samsung usb jig" from ebay for a couple of dollars. It will reset counter and remove warning so if you need to return for warranty, just flash Intratech Official Firmware (removes root too_ and use jig. Another plus, is that the jig will try to force the phone into download mode, and may help recover from a soft-bricked phone. So in any case, very handy to have.

Edit2: Some apps such as the CWM Manager, SetCPU etc require "SU" - SuperUser access, tap allow when requested.

Anyway, that should help you get started,......... happy flashing ;)