Check her Polyvore page for a full list of each product both cheaper and more expensive: http://www.polyvore.com/kristen_stewart_oscar_hairstyle/set?id=17402368
First, start off by washing and conditioning your hair.
Tip: Using non-frizz or moisturizing formulas will help to contain fly-aways. I particularly love the ones listed on the Polyvore. The Living Proof shampoo and conditioner are amazing, as well as the new Naturals line from Tresemme; plus the Naturals line is all certified organic.
Towel dry hair.
While hair is still wet, apply mousse to hair. Then distribute mousse throughout using a wide toothed comb.
Dry hair with a large barreled round brush.
Tip: To get maximum volume, either apply a volume spray while hair is still damp to just the roots, and/or section off your hair while blow drying. Then using the round brush, start at the roots, under your hair, and with the concentrator attachment blow air on to the roots until dry before pulling the brush through towards the ends.
Curl your hair with a 1" barreled curling iron.
Tip: For long hair, use a 1.5" barrel.
Then, once cooled run your fingers through your hair a few times to distribute waves.
Secure off a portion of hair, about 1-2" wide in the very front of your scalp. With that section, make smaller sections (about 4 or 5). Then with those pieces, about halfway down the strands of your hair, loosely braid it to the ends.
Tip: If this is too hard, then you can always twist it, and it will look just as good.
Next, gather all the braids, and the rest of your hair and lightly pull your hair back into a loose bun at the nape of your neck. Do this so there is still volume at the roots, and so that your hair isn't too tight. If your hair is too tight, then more than likely, your overall hair style will look to heavily done, and not effortless like Kristen's.
Then secure it with bobby pins.
Tip: Kristen's hair is in multiple layers due to her Joan Jett haircut, and if yours is in multiple layers too then you will more than likely have a few strands to come loose from the bun. No worries; just take them and secure those down at the base of the bun with bobby pins.
Finish off by spraying hair all over with hair spray, and lightly patting down to secure fly-aways if you have them.