Open the Editor and set the time signature to 3/4. There is generally no need to change the barlines, but a bar holding more than three chords will not play.

 

Then choose a style that suits the song. For a medium slow waltz, try Country, Jazz - Ballad Even, Ballad Melodic, or Long Notes. Adjust the mixer for any instrument that is too loud, and set the tempo by tapping it in.

 

For the full guide, see Accompaniment styles on the iReal Pro website.