It varies a little from sorority to sorority and from campus to campus, but generally when a sister gets lavaliered or engaged, a candle pass is held. The sister will privately tell the chapter president that she needs a candle pass, and with my chapter they were generally held right after chapter meetings.
We would gather in a rather large circle, and a lit candle (fake, unfortunately) would be passed from sister to sister while we sang a Kappa song. The candle would be passed once for a lavaliere or twice for an engagment. (TrueBlueKappa, correct me if I'm wrong cause I can't count!)
When it arrived at the sister who the candle pass was for, she would blow the candle out. (Or in our case, turn the candle off.)
It's amazing how much less romantic a candle pass is with modern technology.