I'm am pretty sure there are many guides in rooting your htc phone as well as flashing a custom ROM. I am also pretty sure that there are guides out there that uses BIG words with not enough Lament words, so here is a simple noob friendly guide to basically make your life easier.
1. Download and install HTCDriver3.0.0.007.exe from http://unrevoked.com/rootwiki/doku.p.../revolutionary
2. Download revolutionary-0.4pre4.zip for windows from http://revolutionary.io
3. Extract it and run revolutionary.exe
4. Connect HTC Sensation to PC and select HTC Sync.
5. revolutionary.exe should detect your HTC Sensation phone and provide your with details about hboot and serial number.
6. select the hboot version and type the serial number into the box and hit enter.
7. It will now try to root phone. Wait and the phone will restart a few times.
8. When it's done, it'll ask to install clockworkmod recovery (cwm). Choose Y for yes or N to refuse. Install recovery based on what the ROM suggests. For ARHD, it suggested EXT4 touch recovery, so press N.
9. Press volume button up or down to navigate and select reboot phone by pressing power button.
10. Download EXT4 touch recovery at http://forum.xda-developers.com/show....php?t=1377759 (thanks to Madmaxx82)
11. Read the Read Me to learn how to install EXT4 Touch recovery.
12. Download firmware 3.32 for the related CID and MID. Be sure to check your phone's CID and MID
13. Download Super Wipe at http://xda.androidrevolution.nl/db_m..._Sensation.zip
14. Download the ROM found at http://forum.xda-developers.com/show....php?t=1098849 (credit to Mike1986)
15. Copy the file PG58IMG.zip to the root of microSD. (root - meaning not in a folder)
16. Copy the super wipe and 6.6.3 ROM zip files to the root of microSD.
17. Power off your phone/ Remove battery from phone
18. Hold volume-down and power button simultaneously to get into bootloader mode.
19. When in Bootloader, it will automatically ask you to update your firmware, press 'yes.'
20. Once the update is complete, immediately take out your battery. (it may sound scary, but it's actually quite safe)
21. Take your MicroSD out and connect it to your pc to remove the PG58IMG.zip and insert it back once completed.
22. Get into boot loader once again (volume down + power button)
23. Now your bootloader screen should read:
PYRAMID PVT SHIP S-OFF RL
24. Select 'recovery' and it will automatically lead you into 4EXT recovery (the one you installed earlier)
25. Install 'superwipe' script.
26. Select 'install from sd card' and look for the Custom ROM you have copied into the root of your Sd card.
27. During the installation you will be asked the following questions:
-Do you have a TMOUS sensation 4g?.
-Do you want to flash the sony bravia engine support?.
-Do you want to flash Stock or Custom Kernel?
-If you picked custom then you will be given a choice between Faux or Sebastian Kernel
-Do you want to flash HQ camera?
Once again this is all your choice, please read up on what each of them does.
28. When the ROM is fully flashed select back
29. Select reboot from 4ext recovery
30. Done, enjoy ICS with Beats Audio, the first boot should take around 5-10 mins
HAVE FUN ROOTING AND FLASHING CUSTOM ROM NOOBS :D
Just remember, do this at your own risk, I will not take any responsibility for you Bricking your phone***