Write the verse and chorus to your song. A bridge needs to build lyrically and melodically from the other elements of your song. .
Write the music to your bridge. Introduce new chords and/or change the rhythm. For example, if the time signature of your chorus is 4/4, you can make the time signature of your bridge 3/4.
Write lyrics to your bridge, adding two or four new lines. Change the perspective and/or reveal a new element to the song's overall theme. For example, if your song's verse tells a sad story and your chorus articulates those feeling in words like, "I'm dying every day," then your bridge could express how you intend to make yourself happy. You could write, for example, "I'm gonna smile every day" to express this.
Write a melody to your bridge. The melody should contain elements of your verse and chorus melodies, while introducing some new notes and chords. The melody should also be a little higher pitched than the chorus and verse melodies, to help boost the emotional energy.
Arrange the bridge in the structure of your song. A bridge usually comes after the second chorus and leads into the last chorus. For example, if you have a solo after your second chorus that leads into the last chorus, you could place the bridge after the solo.