On large areas of unbaked clay just use the side of your thumb or roll with a knitting needle. Many doll makers are using silicone tip tools to smooth small areas. You can use paint brushes too. One of our favorite doll artists said to try a short flat oil paint brush with stiff bristles first. Go in different directions to smooth, don't keep going the same way because it leaves ruts. Then use a small flat watercolor brush to pat down and feather over those areas. Smooth one area into the other. Get the clay as smooth as you can before baking.
Once you have baked your clay you smooth it by sanding and scraping. Hold you work up to a light to find any sharp edges and depressions. Scrape first with a sharp blade and then sand with 320-600 fine grit wet-dry sandpaper. You can sand and scrape after the first bake but be very careful because the clay is extremely breakable at that point.
To sand, use a small bowl of water and dip the sandpaper into it. You'll get the doll very wet, but that's OK. Keep rinsing the sandpaper to clean the debris off of it. Make small pieces of sandpaper by doubling over the edge. Make a small point with the sandpaper if you need to get into a tight area. If the area is too tiny to get into with sandpaper, use small fine files with curved edges on them.
To scrape, it is easier to watch it done than to explain it. This is a video on making a baby but has information on sanding and scraping: