Turn on Chord Scales to see the recommended scale for every chord on a staff. Tap the chart to show the Player controls, tap the Chord Diagrams button (lower right), choose the Chord Scales (staff) option, and pick treble or bass clef. On Mac, open it from the Window menu; on Windows, from the View menu.

 

Tap and hold a bar to see its scale, then swipe through the options to choose one to practice.

 

For the full guide, see Practice scales on the iReal Pro website.