Start with the best-known Nirvana signatures. In terms of all-time popularity, the prize would go to the Emaj-Amaj-Gmaj-Cmaj stompin' chord progression in "Smells Like Teen Spirit," rom the album "Nevermind." Other chord progressions are similarly straightforward.
Go to all-out guitar pounding. The three-piece Nirvana always relied more on power than intricate melodies, from the tracks on 'Bleach' to those on the best-selling "Nevermind" Album. Some of the best evidence of this 'sloppy' style can be found on the bonus track "Twelve" (Nevermind) and the hit "Negative Creep" (Bleach).
Use the single note. In the midst of Nirvana chord frays, a single note on a high string can provide great tension that is signature Nirvana sound. Once again, "Smells Like Teen Spirit" embodies this style as much as any Nirvana song, when the flailing chord progression gives way to the high E string, while crooner Cobain moans "Load up on guns..."
Use interesting Nirvana single-note lines. The bass-type groove on "Lithium" (Nevermind) is a popular one. Also listen to the modish walking line on "The Man Who Sold the World" (Unplugged: A Bowie cover) or the classic up-and-down sequence of "Come as You Are" (also off of Nevermind).
Listen to the songs like "PennyRoyal Tea" on the "Nirvana: Unplugged" album to get a feel for the softer, acoustic tones of Nirvana. Some of these songs are covers, but some are original, proving that the band was not always glued to the adrenaline-fueled garage-rock that made them famous.