How do I turn on course handicaps for players on different tees?

This setting lives alongside Net/Gross in the Format & Scoring step (or an event's Settings dialog).

1. Open the Format & Scoring step or settings tab.
2. Turn on Course handicaps.

With it on, each player's handicap is adjusted using the slope rating and course rating of the tee they actually played, instead of using their stated index as-is. Turn this on any time your group plays from different tees in the same round — it's what keeps a mixed-tee group scored fairly. Leave it off and everyone's stated index is used directly.