Decide where you're going to insert your break. A common pattern for many songs is to feature a verse, followed by a chorus and then a break. Other songs feature multiple verses and choruses, followed by a break. Most songs return to the chorus or feature one more verse and then the chorus after an instrumental break.
Notate how long you want the break to last. The duration of the break needs to fit within the rhythmic scheme of the song. The length of a break usually mirrors the length of a verse or chorus. For instance, if your verses are eight measures, then your break might be eight measures as well. You can either make a written note such as, "instrumental break for eight measures," or you can leave eight measures blank in your sheet music with a note like, "guitar solo."
Write in the notes that you want to use in the break, or leave the section open for improvisation. In the case of piano-driven songs, for example, a song writer might write a break that features instrumental notes that are similar to the melody of the song. If you're working with an experienced band, however, you might want the musicians to create their own solos on the spot (improvisation). The break should remind you of the melody of the song, but most breaks don't imitate the melody note by note.