Do you want your little princess to look good for your wedding?
Steps
Ask your girl what her favorite color is. Maybe purple or pink or orange, etc.
Find a light color, maybe she likes pink for say. Find a light pink, and put a sash around the waist.
After you have that done, you need her to get in it. If she doesn't want to get into the dress at first, promise her something she likes. Maybe play with her afterward, or go out for ice cream.
If she still doesn't want to get into her dress/skirt, well, you are the parent.