1. Go to the song you would like to edit, tap/click on the Edit icon (pencil).
2. Tap/click on the first measure where you'd like the rehearsal mark.
iPhone/iPad/Android: Tap the Rehearsal letter button along the bottom row of buttons (4th button from the left) then choose from the popup menu. (To remove, tap on the space it belongs to, tap the Rehearsal letter button, tap on the Rehearsal letter which is highlighted to toggle off.)
Mac: Click on the appropriate Rehearsal mark. (To delete, click the space it belongs to, the Rehearsal letter button will highlight, click on the button again to toggle off.)
The IN (intro) and V (verse) rehearsal marks indicate a section in the song that will only be played once at the very beginning. Generally only use one of these, once at the beginning of a song.
Subsequent choruses will start after the intro or verse indicated by the presence of an A, B, C or D rehearsal marks (so always use one of these after an IN or V so the player knows where the song will repeat from).
However use of the D.C. (da Capo) will return to the beginning of the song as expected.
In addition, many of the styles will introduce variations when going from the A section into the B,C or D section (most of the Pop and some of the Latin and Jazz styles support this). The dual styles (like Latin/Swing or Swing/Two Four) change between the A and B sections for the first and last chorus.
Generally a drum fill will accompany the transition from one section to another where you have placed a double barline.