For Siyah ICS kernel, see here:-
You will need to be rooted to install Siyah kernels. See here:-
Rooting Galaxy S2 - Dummies Guide
For step-by-step installation and settings for the Siyah kernel, see here:-
Siyah kernels 2.6.7 -> 12 A Dummies Guide to installation and settings
Siyah 2.6.2 - A Dummies Guide to installation & settings
Siyah kernel v2.1 Final
version is now available for download.
Not to be confused with the Dark Knight Siyah 2.1
kernel... there are differences.
updated CWM recovery to 22.214.171.124 and added back soft-key support (thanks to mustaka@xda for the back-key modification. sagol hocam…)
reverted 500MHz deep-sleep enter change for stability purposes
bus frequency modification
For more information and download, see here:-
[KERNEL][UNIFIED] SiyahKernel v2.1 - "One kernel to rule them all" - xda-developers
Both CWM .zip and .tar (for Odin) files available.