Complex sentence: The band played both own songs and covers (The Beatles, Little Richard, The Byrds, Idoli etc.) The most of the year 1991 was spent in composing the music that will appear on the debut album.
Split into simpler sentences: The band played both their own songs as well as cover versions of The Beatles, Little Richard, The Byrds, Idoli songs. Most of the year 1991 was spent in composing the music that would appear on the debut album.