Any pretty edging will do. Just be sure to measure the length where it will be worn, and if you are designing it to lay flat, add a little so it's easy to slip on and so there is some give due to muscles moving when the bride walks. You can design it to lay flat, but that takes careful measuring. The easiest thing is to make it long enough to ruffle up when it is gathered up. You choose how ruffled you want it to appear by how long you make it. This is easiest for adjusting the fit to the bride.
You can either weave pretty ribbon through the design, or add some hidden chains along the back to run the ribbon through. Then hand stitch some delicate elastic band behind the ribbon, using a thread that matches the ribbon color and type. Take care to lay the stitches along the grain of the ribbon so they blend into the material. Without that elastic, no matter how you tie the garter tension, when the bride starts walking, the garter will work its way down to her ankle!
NOTE: Be sure you measure the elastic properly and sew the elastic ends securely. If you don't join the ends securely to each other, the tension will pull in opposite directions at that spot and stress your lace. If you make the elastic too tight, you will cut circulation to the bride's foot. She doesn't need to limp down the aisle with a dead feeling foot, (no matter how funny that could be). When sewing the elastic to the ribbon, you have to adjust the "play" between the ribbon and the elastic as you go along so that the tension is even overall around the garter. When you are done, the relaxed garter looks rippled in your hand.
Most important, have the bride give it a test run in her hose and shoes. Make sure it matches and is the look she wants. Have her wear it a while and walk in it. Stomp, dance. See if it stays put or if her toes get numb. Sounds funny, but you really don't want a problem to show up on wedding day.
And...post a pic when you are done. Everyone would love to see how your finished project turns out.