Improper latch-on can cause pain and really interfere with breastfeeding. Here is a walk-through on proper latch straight from my own lactation consultant, Elisa Hirsch, RN, BSN, IBCLC.
Position your baby with her body facing yours and with her nose close to your nipple. Allow your baby’s head to tilt back so she’s looking up at you. Wait for her to open her mouth widely. When she does, bring her closer to you with her chin in very close to your breast. With your finger, guide your nipple into the top half of your baby’s mouth. When your baby is positioned properly her chin will be against the breast and you’ll see more areola (darker area) showing near your baby’s nose than chin. When your nipple comes out of your baby’s mouth it may look a bit longer, but the shape of the nipple should look the same as it was before the feed. A feeling of tugging is normal, but pain is a sign that you should ask for help.
(Excerpt from Mommy Calls: Dr. Tanya Answers Parents’ Top 101 Questions About Babies and Toddlers, AAP 2008)