Download Android 5.1 Gapps (DO NOT USE older Gapp as you will FC!!!)
For installation, please follow How to Install Android Lollipop ROM on Galaxy S2!
To install, reboot into recovery, make backup ROM, wipe data/factory reset, install ROM, install Gapps, install SuperSU and reboot. (If for some reason you end up in a bootloop, reboot into recovery, go to Advanced Wipe for TWRP or Mounts & Storage for CWM, format/wipe /system, then re-install ROM.)
Credits – XDA <— Please donate to the developer of this ROM or hit Thanks button on XDA if you like it, thx!
Missing root? See How to Enable Root on CM12.1!
Want Flash Player? See How to Install Flash Player on Android Lollipop!
If you want some awesome audio, check out Viper4Audio FX MOD, which you can install on this ROM to enhance your music 100%.
If you want to run your apps in tablet/hybrid/custom DPI mode, see our Xposed App Settings guide.