This information comes from an evening class done by Matt Pelton, twice world champion of Dutch oven cooking. Here's his book:
Matt Pelton, The Cast Iron Gourmet, Hobble Creek Press, Springville, Utah, 2013.
Invented in XVIIth century Holland; brought to England later. The name actually appears to come from the fact that Dutch peddlers widely sold them in the United States off their wagons. Originally, they were big, spherical pots, often with feet, often with lids.
Mormon pioneers reengineered the Dutch oven to make it the flat, modern style. Because Utah's culture is conservative and commemorative, the oven has never lost popularity nor has anyone lost the art.
On the trail, Mormon women began to prepare dinner at breakfast time, place it in the oven, place the oven in the wagon (in the tin basin) with coals. After an exhausting day's journey, they didn't have to cook dinner, but just opened the oven and served.
Choose one whose lid fits tight all around, isn't warped and doesn't have more than a ⅛" of play. Lodge has the highest quality control of all: if an oven's lid doesn't fit, they throw the oven back for recasting.
To clean a very dirty oven, scrub with soap and much water, use a plastic brush, heat hot to open pores. However, do not "burn" the oven on a fire à al Boy Scout. That will cause it to overheat and warp.
Dry the oven by warming it to evaporate the water.
The tale about not using soap to clean an oven is scurrilous and just plain wrong.
Do not scrub the oven with salt as this does to the oven the same thing as salt on highways does to your car.
Rust? Use soap and water, then rub with oil on cloth until clean. If bad, wire brush and start over. Then use the oven, and season before retiring it.
Cast iron is porous with many pits and crevasses. Seasoning is filling in these. Also, much use will burnish the iron smoothing it too, but do not burning, scrape or otherwise mill the oven artifically because it's hard to season such a surface. Let natural aging do this.
Canola and very high heat 500° upside down in a big oven until it stops smoking. Upside down over a cookie cheet to catch the drips. This is because oil begins to evaporate at 350° and then it rises and will otherwise "season" the big oven, which you do not want to do. Re-oil, then gently wash with soap and water to get rid of excess oil. Repeat as desired.
The frequency of the seasoning should match the frequency of use. There is nothing wrong with letting an oven sit for years without seasoning as long as it's not rusting.
Wash and dry before storing.
Store oven with no oil inside in a cool, dry place and a paper towel betweed lid and oven. Rancid oil turns acidic, tastes bad, ruins seasoning. Don't oil the oven after, but only before use.
Metal spoons, forks and spatulas will not hurt the oven. However, do not scrape repeatedly to avoid damaging the patina.
Do not line an oven with anything but parchment paper, especially not with aluminum foil. This only causes uneven hot spots and burns or undercooks food.
There are exactly four configurations for cooking with coals.
Ther is no valid rule of so many degrees for each briquette. This is because briquettes are of differing quality and humidity when bought. This is also affected by altitude and ambiant humidity. Your only choice is to acquire skill through experience. Try to use the same heat medium for best results.
The chimney is best: the coals are ready when no longer smoking. Blue Kingsford are the best and last the longest, maybe an hour. Other brands less.
Don't cook on the ground. The ground absorbs a good deal of the coals' heat leaving much less for the oven to absorb. Better surfaces are metal table tops and concrete.