You can also contact her chapter president and announce that you'd like to come in and lavalier her as a surprise-- you might do this during chapter dinner or chapter meeting at a scheduled appointment time you arrange with the pres so that all of the sisters are there. You can serenade her with a song or read her a poem and present her with the lavalier. Her sisters will totally fall in love with you!
You could also take her on a romantic date and present it to her on the date.
Either way, there is no right or wrong way to give someone a lavalier! It is a sign that you love someone very much, and it is a token showing that you are in a serious and committed relationship.