After finding your course and opening its profile, tap the blue button 'Start Round'. This will lead you to the 'Round Setup' screen, tap on the 'Starting Tee' and select the tee from which you'll be playing.
Make sure that you've selected the correct 'Distance Units' and that your 'Handicap' is correct. Please note that Hole19 will automatically calculate the Playing Handicap for you, it has into consideration your personal/exact handicap and the course ratings, if this information is correct, then the playing handicap should not be changed.
Tap 'Download Map', if it's the first time you're playing in this course using Hole19, and 'Start Round' to begin.