Intel SSD Caching has a capacity limitation of 64GB, so the largest SSD you can use is 64GGB. ASUS SSD Caching has no capacity limitation, so you can use any size SSD.
The instructions for ASUS SSD Caching start on page 4-16 of your motherboard manual:
1.) Shut down your pc and connect your HDD and SSD to Marvel SATA ports SATA6G_E1 & SATA6G_E2. See page 2-2 of your motherboard manual for the location of the ports.
2.) If you have not done so already, install Windows on your HDD.
3.) Install AI Suite II software from the support DVD that came with your motherboard. If you don't have the disk then download it from this link (under "Utilities"):
http://www.asus.com/Motherboards/P9X79_PRO/#support_Download_30
4.) Reboot pc and run AI Suite II
5.) Click "Tool", and then click "Caching Now!"
Once Status shows that initialization is complete you should be done.
Although it's probably not necessary, I would reboot the pc again after initialization is complete.