Neither D.D.'s One Sixth Sense nor the artist has any affiliation whatsoever with any doll company or manufacturer.  
Copyright  2008  D.D.'s One Sixth Sense - All Rights Reserveddeedee@onesixthsense.com http://www.onesixthsense.com
WOW!

No more teeth!
BEFOREAFTER
Profile
Front View
Step 6

You are finally ready to color in your lips!  I usually start with a flesh tone just so that my eyes have a "blank" canvas with which to work.  It isn't necessary, but I like to do it this way.  I use this color to shape the lips exactly where I want the color.  See...looks better already.  Because the base coat is very thin, the lips still do not appear as they will once the final lip color is applied. After this is dry, I add my color(s) and details.  When the paint is thoroughly dry (I usually wait at least 2 days), I seal the lips with a high gloss sealer/varnish.
"Looks like I've been drinking chocolate milk!"
Step 5

This is an optional step but one I usually do take.  Seal the entire lip area, including the vinyl outside where the Gesso was brushed on.  The matt varnish not only seals & smoothes the Gesso but will seal the vinyl where the lip color will be.  I do this because, in some cases, dark red and orange acrylics can bleed off the lips.  In most cases, you want the paint to stay where you put it!  After sealing, I wait at least 24 hours before moving on.  I warned you that this was a tedious process!
Ooooooo!!  That tickles!
Look closely & you can see that there is now
a slight indentation where the lips meet.
Step 3

Okay, now that you've slightly overfilled the gap with Gesso and the layers have dried thoroughly, you can now begin the shaping of the lips.  You'll want to use a light touch with the nail board.  Make sure you have a very fine grain on it.  You can always take off more of the Gesso, but you won't want to have to go back in with your brush to replace what you've accidentally taken away.

Start by simply smoothing the surface of the Gesso with the nail board.  Once you have removed the bumps and imperfections, I suggest you take a few moments to study the lip area and decide exactly how you want the lips to be.

You can see 3 different types of "files" in this photo.  Choose the one that works best for you, or...if you're like me, have ALL of them on hand!  The pink ended one actually has the sanding element on both ends of a thin dowel.  These are chiseled at the ends too!  They come in different grains and can be found at beauty supply stores.
Step 2

This is one of those instances when it's going to get worse before it can get better.   For at least the next few steps, your doll is going to look like she's been drinking milk from a bowl without the benefit of a spoon.  Don't worry.  She'll eventually get cleaned up and look presentable.  With your tiny paint brush, you're going to paint in the first layer of Gesso.  The goal over the next few steps is to build up the Gesso until it fills in the depression where the doll's "teeth" were.  Try not to get too messy with the Gesso.  You do not want to paint the Gesso outside of the lip line.  Try to picture in your head where the filled in part will meld with the existing lips.
Here are some side shots so that you can see how we are building up the Gesso so that the new surface will match up with the vinyl lip areas.  This is a time consuming process.  You don't want to rush it.  Many thin layers will look nicer than one or two thick layers.  You have more control.  The goal here is to slightly overfill the gap because we will be sanding and shaping later.
Don't worry!  It WILL get better!
Step 1

The first thing you need to do is remove the doll's lip/mouth color.  If you're going to do a full repaint, you might as well take off all the facial screening.  I use straight acetone and cotton swabs.  I buy my acetone at a beauty supply store.  It works well without causing me any stress.  Stress causes wrinkles.  That's my story, and I'm sticking with it.  After removing the face paint, I wash the vinyl with a gentle soap and water.  Pat dry.  I've been told that this cleansing/neutralizing step is unnecessary; however, it gives me peace of mind, so I do it.  You can choose to do it or not.  It's your call. 
A Clean Slate
Items Needed:
Smiley Faced Doll
Gesso
Small Paint Brush
Fine Nail Board(s)
Pencil
Tapestry Needle (optional)
Matt Varnish/Sealer (optional)
Pretty, but not much personality.
Supplies you will need.
D.D.'s
for the diminutive diva
One Sixth Sense
and OSS Unlimited for Doll Art in 1/4 Scale
One Of A Kind Doll Art
Thank you for Shopping with Sense!
OSS Tutorial
Closing a Smile - Method #2:  Filling It In
Step 4

Do you want a fuller lower lip for a slightly pouty look?  Do you want lips evenly matched in fullness?  You get the idea.  Once you've decided upon lip shape and size, pencil in the line between the two lips.  (You can see my pencil line in the photo in Step 3) Once you have the line, begin to round the upper and lower lips into the center so that you will have a slight depression.  And please don't worry about the white part of the lip line.  This is NOT the line your color will follow! Your doll might look like a clown at the moment, but never fear...she will soon look maahhhhvalous!

You can also use a tapestry needle to deepen the pencil line, but if you do this, be VERY careful.  Do not press in too hard.  Remember...it is better to do this step several times to deepen the line rather than digging in too deep and then regretting it.  I use the needle to make a shallow groove where my pencil line is, and then I use one of the chiseled end files to lightly sand along the edges of the groove.
You can start to see the lips taking on some definition.
Ho Hum
WOW!!  What a beauty!