1. Install Ragnarok Online into your PC. I suggest use KRO Sakray as the files are more complete then the rest. However, any RO client should work just fine.
   
2. Once you've installed the RO client, click on the game the allow itself to update the program. NOTE: You MUST be connected to the Internet to allow this update. The patching process takes about 5 - 20 minutes depeding on your RO version and connection speed. Rest assured that you only need to do this ONCE (unless you re-install a fresh client or a critical update is necessary).
   
3. Once the update is done, EXIT the program (unless you intend to play the official server, then hit the START GAME button, but I don't think you'd wanna do that since you're here to play TRINITY server in the first place. Sheesh!)
   
4. Unzip the Trinity8-0.zip and copy all the files (12 files total) within the Trinity folder and paste it inside your RO folder in your computer (e.g. "C:/Program Files/Gravity/MRO/").
   
5. Once that is done, you can now play your Ragnarok Online by clicking the TRINITY.EXE icon. Create a shortcut on your desktop so that you won't have to open your RO folder everytime you wanna play.
   
6. You should be able to login with your registered ID after that. If you haven't yet register, then what are you waiting for? Register HERE
   

Not enough guide for you? Check out these links then:
Another installation guide (with screenshot!)
Frequently Asked Questions (FAQ)