This is a very sweet idea for those that love music and have that artsy side. You can pick songs that have meaning for the two of you, a song you can laugh to because it reminds you of when you saw her at the party dancing in that cute funny way, or it can be a song that the both of you have listened to over and over again together.
The songs in the CD can be your collection that you want to share with her. It can also be a collection of love songs that express deep feelings for the person, making a commitment, songs that talk about getting married, making the perfect introduction to your proposal message.
If you sing, or play an instrument, this would be a nice time to record it on the CD and dedicate it to her. After the collection of songs, you will have a special message dedicated to her. You can record your proposal on the CD, or have her listen to the entire CD, then propose to her in person.
You can also get creative with the CD cover. You can print a picture of the two of you and write a special message: "This is for my love" or "I have something to tell you." You also have different choices for how she will receive the CD. You can simply give it to her or make it more interesting by having her find it in her car, in her purse, on her bed, at work. Anywhere really!