install cyanogenmod android lollipop 5.1.1 on galaxy s2 (i9100/i9100P) step by step

install cyanogenmod android lollipop 5.1.1 on galaxy s2 (i9100/i9100P) step by step

Before do anything you should know the difference between nightly, snapshot & stable Rom:

Stable is after all the features desired by the CM devlopers have been put in and the code has been tested.

 Snapshots (M builds) are builds done roughly once a month that has had some testing to make sure things are mostly working and released to the public. These are the first builds that allow official bug reports on the project manager site.

Nightlies are automated builds that are built, well, every night. There is no human interaction with these and they are largely there just to see if the code added throughout the day will compile. As far as the devs are concerned, there are no bad builds with the nightlies, because if it doesn't build, that is news to let them know that something is screwed up in the code. They don't accept any bug reports on these builds.

Generally, the nightly builds, while extremely experimental and considered bleeding-edge, tend to be relatively stable and mostly bug-free. Many people use the nightlies as their so-called daily-driver, meaning that any bugs that they may have aren't so severe that they want to switch to a different version.

1-Avoid to install Nightly roms, because in some versions you will not be allowed to enter to download mode if somthing bad happend.

2-if something bad happend with the installation (be sure problems happend if you don't fellow instructions), you can solve the problem by installing stock rom with Odin.
Links to stock roms:
-i9100......... - 570.54 MB - 556.19 MB
-Odin flasher  Share Odin_v3.07

How to install cyanogenmod?

first step: install CWM Recovery
-If you have a cm11(kitkat rom)installed on your galaxy s2, you are good to go & no need to install a CWM Recovery again (it's compatible with cm12 (lollipop rom).
but avoid to install cm12 by OTA Method because the no comptability of apps.

-If you are using a stock rom, install CWM Recovery manually using Odin:
                        1-Download Compatible CWM Share CWM6.0.4.5-KitKat lollipop-i9100.tar - 5 MB
                        2-Download odin  Share Odin_v3.07
                        3-choose pda & click star (check only Auto Reboot & F. Reset Time)
                        4-Don't touch anything till the phone get fully on.

Second step: install cm12 & google apps via Recovery:
                        1-download cm12 latest release

                        2-Download google apps  
                              Share tk_gapps-modular-pico(uni) - 49 MB
                        3-Copy cm12 rom & apps zip files to phone (internal or extarnal memory card)
                        4-Enter recovery mode
                               *CM11 users go to sitting/ developer tab/ enable advanced reboot
                               *others shut down phone (home button+ volume+ + power button) or download                                      any app from google store allow you to go directly in recovery mode.
                        5-Do factory wipe & Wipe cach partition. go advanced and do wipe dlvk.
                        6-don't reboot your phone, go install zip file from sd card ( install cm12 rom then                                     instal google apps). reboot your mobile wait till the installation complete.