You will want it to be memorable and perfect as well as suited to your girlfriend. Here are some ideas: go to the place you first met or where you had your first date; hang the ring in a present on the tree at Christmas-time; hold a scavenger hunt, with the ring at the end-point: hang a banner a window at work or from her apartment: pay a street busker to sing ``your song´´ as you walk past and then drop onto one knee in the street: write in the snow or beach sand.
Seeing as you are thinking of asking this important question, you now probably know your lovely girlfriend pretty well by now. You know what she likes, and the places she loves to be, and you have already made a lot of memories together. So, you could choose one of her favourite places, like the beach, or the mountains, or the park, and just when she is not expecting anything you drop down on your knee and bring out the little box with the ring! Or if she likes reading, you could buy her a book by her favourite author and conceal the ring inside the book and wait for her to find it while you watch quietly. Maybe she loves playing Trivia games, so you could invite family and friends for a games night and then at the strategic moment you present the card to your girlfriend with the important question on it!