Score the outer skin of the chestnuts - I recommend making an "x" on the flat side of the shell, as large as you can but without cutting the nut inside. Discard any nuts that have holes in them or are mouldy.
As you score the nuts, place them in a bowl of clean water and move around gently to clean the shells.
Once you have scored and cleaned all of the nuts, take them out of the water, shake off excess water slightly, and place in a baking sheet with the scored side facing up.
Roast the chestnuts for around 15 minutes, larger ones will take slightly longer, smaller ones may be less. When ready, the slits you made will open up and you can test the inside nut to see if it is tender. The skins may start to brown slightly but should not burn.
Once ready, tip the nuts into a clean dishcloth and wrap them up. Leave a minute to be slightly less hot, then take one out at a time and peel off the shell, including the inner shells, using the opening made from your slit to get started. Try to work quickly as they are easier to peel when warm. Use a pairing knife to get out any stuck pieces of inner shell, as needed. You can eat these room temperature but they are probably best snacked on while still warm.
Note you can make larger or smaller quantities, as suits with the same process. If roasting ahead of using in a recipe, note you lose around ⅓ of the weight through removing the shells.