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Are you talking about recording or mixing? If it's mixing, there's a great EQ guide by Cdra: http://utau.wiki/tutorials:equalization:a-mixing-tutorial

I meant for UTAU covers. Suppose I want to cover a song that doesn't come with an off vocal (such as Teto Teto ★ in the Wonder Night) and I want to create an off vocal for it, but every time I do, the results turn blurry. Do I need a specific program that eliminates all vocals on a song?

That's pretty complicate ^^" There are methods, but it mostly depends on luck on how the song was mixed.