I got this from from Thehairstyler.com. It wasn't the exact same pic of the Olsen twins, but the hairstyle was pretty close.
1: Apply Mousse
Apply styling mousse to damp hair, using your palm as a measuring guide, and evenly distribute it along the hair shaft. Styling mousse will add hold and can help achieve better results.
2: Use A Large Radial Brush
A large radial brush is usually used on long straight hair types or to smooth out wavy or curly hair textures. When selecting a radial brush, always use one with a pure bristle as this will not tear or damage the hair.
3: Blow-dry The Back Under
To blow-dry under, start at the nape/back and place your brush under the section of hair close to the nape. With the dryer held up over the hair, pull the brush from the roots and continue to the mid length?s, keeping the blow-dryer following over the top. Once you reach the ends, curve them under. Repeat this step until the section is completely dry.
4: Blow-dry The Sides Under
To blow-dry the sides, take out a small section of hair from the front and clip the rest of the hair away. Next, place the brush under the section and then pull the brush and hair forward around your face (this curving motion creates body). Place the hair-dryer on top of the brush until you reach the ends and then turn the ends under. Repeat until dry.
5: Part The Middle
Using the corner of your comb and standing in front of a mirror find the middle of your nose and then drag the comb straight up slowly until you reach your hairline and then continue back in a straight manner to achieve an even middle part. A middle part is a great way to make small, round face shapes appear longer and thinner.
6: Use Large Hot Rollers
A large hot roller is used to give your hair a larger curl and to add lots of body.
7: Apply Hot Rollers To The Side
Take a section of hair no wider than the diameter of the hot roller and comb it through to ensure there are no knots. Place the roller at the ends of the section of hair, being sure to smooth the ends under, and then twist the roller vertically, making sure to roll it backwards, along the hair shaft to the roots and secure it with a pin or clip. Keep the rollers in your hair until they cool.
8: Apply Smoothing Shine
Apply a very small amount of smoothing shine to the palms of your hands and then run it through the mid-lengths and ends of your hair. Be careful not to add any to the roots or a large amount to any section of the hair as smoothing shine can be very heavy on the hair, weighing it down and making it appear oily.
9: Apply Lacquer
Lacquer is great for strong hold. A small amount is all that is required to finish and should be applied from an arms length distance to the entire style.