Making a fire
There are many kinds of fires - from bonfires to deep earth ovens and roasting pits. I'm going to be concerned with an all-purpose fire that serves as heating, lighting and cooking stove.
It's a good idea to remove the turf before building the fire, so that you can replace it when you leave and help the grass to grow back more quickly. Your fire pit doesn't need to be bigger than 2 - 3 feet across unless there's loads of you.
Make a ring of stones to surround your fire, which will insulate it and retain heat, keeping you warmer if you sleep outside. Stones help a lot when you're trying to balance a pan over the fire, and they allow air to flow underneath it, keeping the coals hot. They also make excellent tea stands (but careful with those plastic cups). Use lumps of concrete or porous stones if you can, because flint rocks can shatter if they get too hot ... but please don't take them from the roadways. If you can't get your hands on any suitable rocks, then just remove the turf and clear the ground of stones, twigs, leaves and stranded earthworms. Some farmers don't like you making fires on the ground at all, in which case you'll need some kind of makeshift brazier or barbecue - an old car wheel-rim is ideal. Any piece of sheet metal, raised off the ground with a rock at each corner, will do. Make sure you're building the fire on a dry surface. If it's too damp, put down a few sheets of newspaper or scrap cardboard and build your fire on top of that.
You need three things - tinder, kindling and logs. Tinder is anything which ignites with just a spark or a match, such as paper or dry grass. Kindling is small pieces of wood like twigs and thin branches, which burn quite easily. It's good to have a variety of different thicknesses, from almost match-thin twigs, to branches about half an inch in diameter.
Make a small pile of kindling in the center of your fireplace, then stack thin kindling all around it in a teepee formation, with the twigs balancing against each other. Continue building up this shape with thin branches, making sure there's room for air to flow through and around all the wood.
Light a small twig and reach inside the teepee to light the tinder. As it burns, it will begin to ignite the kindling, and the fire will come to life. If it seems sluggish, blow gently to encourage it.
You're ready to put some logs on now. Having a variety of different sized pieces helps, because you can start by adding thinner ones and building them up as the fire gets hotter. But basically, just chuck it on, paying attention to the airspace between logs. A bit of blowing here and there always helps.
Keep the fire going for half an hour or so, and get a good solid bed of hot embers going. Embers are what you need to cook over, because they give a reasonably constant high temperature. If you're cooking at night and the fire is your only source of light, it helps to kind of divide it in two - keep one side heavy with logs and flaming all the time, and keep dragging the embers over to the other side for cooking.
If you happen to have a broken washing machine, the drum makes an excellent barbecue. Turn it with the hole facing upwards and then chuck some hot coals into the bottom. You can put a griddle on top and use it that way, or a wok is just the right size to sit in the hole and cook up a stir fry or pan fry some fish. In Cornwall we had fresh mackerel cooked like that. Lovely