Alright, so here is my guide that I figured out all by myself - trial and error. Yes, I know there is a similar guide on the internet, but I didn't copy - read previous comment in bold.
Anyway, to start, you need to have the BGM music turned off, which can be done in-game in the options panel (altl O). See below for a screenshot if you are too stupid to understand.
Next, you need to navigate to your BGM (Background Music) folder which should be located in your Ragnarok folder. Again, see below.
Go into the BGM folder and listen to all the tracks - it's fun and nostalgic! *Ahem* Anyway, find the song that you want to replace with your custom tune (for example 08 is Prontera, 01 is the training grounds and so on...) Take note: Below we will be working with the Log on (and Training ground file) which is, as you can see, called 01.mp3. see below.
What you need to do here, is rename the file, so that you remember what it was originally for. Here, I renamed mine to be "01 TRAINING.mp3". This way, I will know that the song was originally for the Training ground and was called "01". See below
Now, locate the song that you want to replace the boring Ragnarok music. I have chosen my favorite emo song lol ^^
Coheed and Cambria - Suffering. COPY and PASTE the song into the BGM folder. See below.
Finally, refresh the page by pressing F5. You will now see the original BGM for the map (which you renamed EG: 01 TRAINING.mp3) and you will see your custom track which replaced the original BGM track (EG: 01.mp3). See below.
ow, go back to your char, and in the options panel (ctrl O), press BGM back to 'on'. You can now enjoy your fresh beats and be jammin' to cool tunes instead of childish RO noise.
Note: People must be aware that not ALL of your songs will work. Songs must be in the .mp3 format, and not too long I guess. Even some .mp3 songs will NOT WORK. I have no idea why this is, but just choose a different song - don't come crying to me because 'my guide didn't work...'
I hope it will be a good help if ur bored , and also i hope u appreciate my workking guide. ol ^^