When you propose, put the ring on her finger. Don’t just open the box, don’t hand it to her. Put it on her finger. It’s all come down to this moment so take your time, don’t drop it, take her hand gently, breathe, and slide it onto her finger.
Not just any finger, but the ring finger of her left hand. You can show her the ring before or after you ask but, only once she says “yes” do you put it on her finger.
She’s waited for this moment, take your time with it and enjoy it. She will wear that ring for the rest of her life, place it perfectly. She will look at her ring often and think of you. You need the perfect ring for her and it all comes to that moment when you put it on her finger to stay forever.