There are a few things an intrepid open-fire cook needs.
One of them is a fire. Eighteenth century cooks would have cooked at home in either stoves (common in England and the Continent) or a hearth (common in the colonies and among the lower sort overseas). Though the colonists were looked down on a bit for their insistence on sticking with the hearth long after it went out of fashion overseas, they had their reasons--plenty of available firewood, so why switch?
Out on campaign, however, you don't have the niceties of stoves, hearths, or chimneys. You have a fire pit.
A good fire pit should be a nice, easy six inches deep at the least, more to cultivate good coals. I prefer an L shape, so that there's a main cooking area and a baking area, but that's my personal taste. Regardless, you get a hole dug in the ground large enough to build a fire in. This weekend I had to deal with a shallow scraping of maybe three inches as we were camped on what used to be a gravel road, whereas a friend of mind dug a trench that you could have easily buried someone in.
Then you need some irons. There has to be some way to set your cooking surfaces over the fire without putting them, actually, in the fire (though once in a while that's not the worst idea--boiling water, anyone?). This can be a pair of upright metal poles with a spit suspended over the fire from which you can hang pots from metal hooks. It can also be a series of spiders and trivets, little metal stands on which you can place your frying pans. I prefer a grate, something akin to the top of a modern grill on little legs that's placed across the fire. You'll also want a poker--to move the logs around in the fire (proper log-spacing is vital to a good cookfire--fires need to breathe!) and a blow tube or bellows, to force air at the fire to get it going.
And what to cook in? Cast iron pots and pans, not so disimilar to the modern ones many people still use in their kitchens today. One name might sound familiar--a Dutch oven. Today Dutch ovens are essentially casseroles, but in the eighteenth century they were large flat-bottomed pots on legs with rimmed lids. You place items to be baked--such as breads, pies, delicious if not entirely eighteenth-century authentic cobblers with canned pie filling (YUM)--in the pot, set the pot over a pile of coals, and pile more coals on the lid. The trick for cast iron is keeping it well-seasoned--properly maintained cast iron actually has a non-stick surface. Non-properly maintained cast iron acts like some sort of insane Gorilla-Glue-esque bonding device and is impossible to clean. An ounce of prevention, people--oil the inside and bake it for an hour or two once in a while. I try to season my iron after every event.
And finally--a fire. Though most people would identify the ability to start a fire as a necessary eighteenth-century skill, keeping a fire going is also important. Starting a fire is not only a pain, you can't do much with it cooking-wise right away, besides slowly boil water. We start the fire at night when we arrive on site Friday and hope to bank it well enough at night that we still have some live coals in the morning--it makes starting the fire up again much easier. And, if you're cooking, you want to cultivate coals--not just flames. Better, more even cooking, and the necessary coals for your Dutch oven.
So--the basics. Yummy food to come.