Start now. Get some pictures of the dress you are thinking of giving her, or get pictures of a couple of different styles for her to choose from. Get some samples of different tartans you think she would like. Tell her you are taking her out to dinner. Then take her to a special diner. During the dinner tell her that you need her help on a special project. You have a special present you want to give her on her birthday, but you need her help.

Give her a nice birthday card with the picture of the dress and tartan. Make sure she knows that this is a custom hand made dress. Since the process will take a while you need her help.

Once she has agreed make each step a special event. When you send the measurement out give her flowers to commemorate this step. When/if you are called saying the dress has been started take her to dinner and give her the email or card saying the dress has been started. When you are told that the dress has been mailed - more flowers. Once the dress comes in take to dinner with some special friends to show her off (excuse me shoe the dress off).

then on her birthday give her a sash or some other item is the same tartan as her birthday present.

Make the present and process special because she is special.