Download Android 5.1 Gapps (DO NOT USE older Gapp as you will FC!!!)
To install, reboot into recovery, make backup ROM, wipe data/factory reset, install ROM, install Gapps, and reboot. (If for some reason you end up in a bootloop or get force-closed keyboard error on boot, reboot into CWM recovery, go to Mounts & Storage->format /system. For TWRP, go to Wipe->Advanced Wipe->Wipe /system.)
Want Flash Player? See How to Install Flash Player on Android Lollipop!
Want latest Android L Preview Keyboard? See How to Install Android L Preview Keyboard on rooted Android!
Want to enable all apps for Multi-Window? See How to Enable all apps for Multi-Window!