Possible pitfalls: Copyright restrictions, so advising to use legal sources. Also, if they're downloading from a site that has illegal ringtones, that's not recommended. Emphasize purchasing or using licensed versions.

Also, the song is known for its catchy tune and has been widely covered and used in various contexts. The instrumental version is popular because of its memorable melody.

I should also check if there are any existing instrumental versions available on platforms like YouTube or Spotify, and include that in the guide. Maybe note that some streaming platforms allow you to play the track and then download it, or offer a "ringtone" version in the track's options.

Now, I should outline what a guide would typically include. First, an introduction explaining the song and its significance. Then, details about the song itself—lyrics, composer, release, and popularity. Next, how the instrumental version is different, where to find it, how to set it as a ringtone on different devices. Maybe also include some background on the movie Karz and the cultural impact of the song. Additionally, there might be legal aspects if someone is downloading it.

Wait, I should check if "Ek Hasina Thi" is from Karz. Let me confirm. Yes, it is. So the song is from the movie Karz. The instrumental version is a version without the vocals, just the music. So the user wants to know how to get that as a ringtone.

Another point: if they're downloading from a third-party site, warn them about copyright issues. Recommend legitimate sources. Also, mention that if they have the song in their own library (like from a purchase), they can use that to create a custom ringtone.