Follow standard outdoor cooking practices when cooking with cast-iron over a fireplace. Never put the iron into the flames, only on the coal bed or just above the coal-bed (rack). Depending on the type of food cooking, you may wish to have a lid on the cast-iron to keep the foods moist-n-tender.
Try not to stir up the coals too much once you start cooking the food.
When cooking with a dutch-oven, spoon hot coals to the top of the oven - the lid - when you are cooking. It will help create a better heat. Dutch-oven cooking is great for virtually any recipe you wish to try that is oven-based. Cookies, cakes, breads, buns, stews, chillies, roasts, chicken, turkey, etc. I have been cooking with my dutch-oven for years and recently purchased 3 recipe books specific to DutchOven cooking full of tricks, tips and hints for cooking all your favorite recipes.
You can also practice stove-top dutch-oven cooking as well as oven-based dutch-oven cooking (follow normal recipe for oven baking and just toss it all into the dutch-oven first).