Sienna Miller's look is perfect whether you are taking it by day or by night, as it has just enough casual flair to take it to work or on errands with you, and the same amount of elegance to don it at an evening event. To get this look, you may want to start with second day hair as this will give you some more bend and make your hair easier to work with.
Create a deep side part, and pin one section to the side while you twist the other. On your other side, start at the front and by sections, twist your hair in a neat chignon so that it lays flat against your head. When you are finished twisting, secure this section at the back with some bobby pins.
After you do this, pull a section into your front so that you get the sexy sweeping bangs look that Sienna has here and run a flat iron over it to make it nice and straight. Then,take the other section of your hair then and wrap it into this section, and roll it into a sexy bun that you can pin at the nape of your neck. You may need to add a few extra bobby pins to secure the look, and finish the look with extra hold hair spray.