Filling chicken breasts seems simple enough -- until the fillings melt and leak out into the pan. Three common errors are at the root of this problem -- too small a pocket, too much filling and warm ingredients. Fix those issues and you'll likely meet with success.
Buy regular chicken breasts, not fillets or tenders. These cuts are too thin and won't hold the stuffing well. Unwrap the chicken breasts and pat them dry with a paper towel.
Set the chicken breasts on a cutting board. Use a sharp knife to make an opening midway between the top and bottom of the meat. Slide the knife into this opening and move it back and forth to make a horizontal pocket.
Sweep your finger into the pocket to remove any attached fibers and to slightly enlarge it so it will hold in the filling.
Poke a small amount of stuffing into the pocket. Use your finger to distribute it evenly. Use no more than 2 or 3 ounces of stuffing. If you overstuff the chicken breasts, the stuffing will almost certainly leak.
Place stuffed chicken breasts on a baking sheet and put them in the freezer for 10 minutes. This quick chilling firms up the stuffing so it's less likely to melt and leak out during cooking.
Insert a toothpick into each chicken breast to keep the pocket closed. Remove the toothpicks before serving.