I agree completely with everything Derby said. Be honest and straightforward, and don't go in with any preconceptions of what shape any new relationship or relationships has to take. Don't expect this woman to complete your lives or move in with you any time soon or love and desire you both equally... just tell her you're interested in being more than friends and give her a chance to tell you what she thinks and feels, then stay flexible from there and go slow.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.