Here you go, this is one of Vicki Peters 'Tips of the week'
Cutting the smile line out with a tip is another trick I have picked up in a
nail competition. Place the white tip product on the nail and let it set
about 60%-75%. You must use a full welled tip with a nice rounded top edge
that will resemble the shape of the smile line you want to achieve when
turned upside down. Size the tips to fit each nail and set them out on your
table. With the tip you have already sized out to match the nail, apply the
white tip powder on one nail. When the product is dry enough dip the edge of
the tip's contact area (holding it with the tip's edge up and the contact
area towards the table (another words upside down) into the dappan dish of
liquid monomer. With the tip wet but not dripping, like you would a cookie
cutter, place the rounded edge of the top of the tip at an angle, in the
white tip powder where you want the smile line to be Cut out the smile line
by pressing the tips edge into the white tip powder and cutting out the white
product. Wipe it away from you. If the product is too wet you may have to
use a new tip and do it again, so practice and know your product when it is
dry enough to do this perfectly. You may want to finish the smile line for
a more perfect shape by wiping it with your brush after cutting it out with
the tip. Sounds complicated but it really is simpler that building a smile
line and wiping it into shape.
Loads more tips here
http://www.beautytech.com/nailtech/vicstips.htm
Have fun with it.