Download all files, then put them all together to a certain folder. Simply double click file, follow the installation process, and point it to one of your drives (Preferrably Drive C:\)
Since data.grf is one huge file, extraction should take a while, you can leave it for a few minutes.
After successfully extracting it, you can proceed to that folder (C:\Rhythm Ragnarok Online as our example).
Double click your RhythmRO Patcher to check if there are new updates.
Now the only thing left to do is for you to register.
You can do that by clicking Register Link.
After doing everything above, you can click the Start Game on your Patcher, and should be ready to play!