Have a look at the cyanogen website and see they have to offer the have a pretty long old list of how install it on a whole range of phones. Its a pretty similar for all phones. Case of enabling dev options, getting a bootloader enabled on your phone. Installing a 3rd party recovery. Which can then allow you to flash a new ROM onto your device. in your case; cyanogen 12.1. You will also need to download a dev program onto your PC that will work with the bootloader to help install the custom recovery.
There's been no official release as far as I am aware of for your phone, which is probably why you're not really finding too much to help you in terms of that sort of thing.
I would use a mixture of this site here to download your files.
http://www.asus-zenfone.com/2015/07/rom-cyanogenmod-cm-121-for-asus-zenfone.html
and then a use of the guides on the cyanogen mod wiki and find a device that is going to be most similar to yours here
https://wiki.cyanogenmod.org/w/Devices
Just beware that as CM 12.1 has a number of known bugs on your device that have been listed on the first website have a read through them and see if they will put your off or not first.
If you think you'll be able to put up with any of those potential bugs make sure that once you have installed your third party recovery to make a backup of your currently installed OS so if all goes wrong with CM12.1 you are able to revert back to it without much hassle.
I think you're going to find your self working between a number of websites and patching together different guides to allow you to be able to install CM12.1 on your zenfone 2 but it is doable.
Try these links also:
http://www.xda-developers.com/cm12-1-on-zenfone-2-closer-to-what-should-have-been/ (there are links on that site to follow which will help)
http://www.dayherald.com/asus-zenfone-2-cm-12-1-rom-custom-build-available-you-can-unbrick-your-device-and-fix-bootloop-issues/4541/2/
http://www.idigitaltimes.com/asus-zenfone-2-cm-121-rom-download-and-install-build-replace-zenui-461177
https://www.youtube.com/watch?v=Vb3CwaIr45k (this video seems pretty handy)
http://www.droidguiding.com/install-cyanogenmod-12-1-custom-rom-android-5-1-1-on-asus-zenfone-2/