Instructions (For Custom ROMs which are custom bootanimation ready):
1. Transfer the bootanimation.zip file to your SD card.
2. Use any Root Browser app and copy the bootanimation.zip and then go to directory /system/media and replace your old one with this one.
3. Reboot and you’re done.
Instructions (For Stock ROM users):
1. Follow guide here.
2. After you have done that, transfer the bootanimation.zip file to directory /system/media as mentioned above.
3. Reboot and enjoy.