In XDA Developers, the bible of developers and restless minds, one of its most respected users, torxx, has published the steps to take to first unlock boot of the HTC One S (bootloader), change the default recovery, become superuser of the terminal (root) and, you can install (flash) any modified version of Ice Cream Sandwich (ROM).
The first step will always be to unlock the starter system. For that you have to go to the official page for HTC developers. In the section Bootloader explain the commands to use. They don’t appear for the HTC One S yet, so go to the All Other Supported Models category.
In a second step, tap modify recovery. You have to download the latest version of the Android SDK if you don’t have it yet and, in parallel, download the Interim CWM Touch Recovery specific to the HTC One S from here. Then you have to place the CWM file in the folder where fastboot.exe is. Now you have to run cmd on your computer and navigate to the folder where the CWM file is. Then you have to write the following command: fastboot flash recovery name_of_the_recovery.img.
In a third step, the turn to root. To become a superuser you must have installed the SuperSU and flash it with the Recovery: turn off HTC One S once it is completely off, press and hold the volume down key and the power key to start the Bootloader. Once there, you have to go to Recovery using the volume and power keys, go to install zip from sdcard, first, and choose zip from sdcard, later. We will see a file named CWN-SuperSU-v0.87.zip. Just select and install it.
It only remains to restart the mobile and choose the ROM that we like the most.
All the details in XDA Developers