Summarize:

You do not need a very large fire, but you will need to have flames and a grill rack positioned just above them to roast chestnuts this way. Place a log into your fireplace, fire pit, or charcoal grill and light it. Use newspaper as kindling to get the fire going. A small amount of lighter fluid may also help to ignite the log. Rinse the chestnuts under warm running water for a minute to clean them. Then, cut an X into the round sides of the chestnuts with a sharp paring knife. Make sure that the X goes through the shell and into the chestnut meat. Cutting an X into the chestnuts is called scoring the chestnuts. Use a large sheet of heavy duty tin foil with several small holes poked into it, a cast iron pan, or a slotted grill pan meant for cooking over an open fire. Position the chestnuts with the X facing up and spread them out in a single layer. If you want to poke holes into a piece of tin foil to roast your chestnuts, use a skewer or small knife and poke a hole in the tin foil about every 1 to 2 in (2.5 to 5.1 cm). Roast the chestnuts until the shells are blackened, which will take about 20 to 30 minutes depending on the heat of your fire. Put your foil or pan onto a grill rack just above the flames. The flames should be licking the bottom and sides of the foil or pan. Watch the chestnuts closely so you will know when they are done. If possible, keep the fire burning at the same level and avoid getting the fire too hot or the chestnuts may burn. After the chestnut shells are blackened, remove them from the fire, and let them cool before you begin to peel them. Use your fingers to peel away the shells around the scored area of the chestnuts.  Make sure to use a fireproof silicone oven mitt to remove the chestnuts from the fire and set the pan onto a potholder or trivet to cool. Place your cooked and peeled chestnuts into an airtight container, and store them in the refrigerator for up to 4 days.
Get a fire going in a fire pit, fireplace, or grill. Wash the chestnuts and cut an X into each 1. Spread the chestnuts out on a piece of tin foil or a heavy metal pan. Roast the chestnuts over the fire for 20 to 30 minutes. Let the chestnuts cool for 5 minutes and then peel them.