Bring some water to a boil in a pan, add your acorns, including their shells & boil them for around 20 minutes.
Leave until cool enough to handle, before peeling the shells from the nuts. Hitting them firmly with something heavy usually helps. Discard any that look bad or have worm holes in.
Roughly chop the nuts, or run them through a food processor, if working in a modern Kitchen.
Roast the nuts until dark brown in colour. This can be done in a modern oven at 200 degrees Celsius, for 20-30 minutes or in a pan or skillet over a campfire. Leave to cool and then grid the acorns to a fine texture using quern stones or a pestle and mortar. In a modern Kitchen you can use a coffee grinder. Store somewhere cool and dry until ready to brew.
To make a cup of coffee, line a cup with a fine cloth like a muslin, or cheese cloth. Add 1-2 teaspoons of the coffee and leave to steep for several minutes. Lift the cloth from the cup, removing the acorn coffee powder.
Enjoy! Some people like to add milk and sugar, but its quite acceptable on its own.