A buddy burner is a type of portable improvised stove, made out of an old shallow tin, corrugated cardboard and wax. These instructions are drawn from the Guide Handbook, 1968 edition.
To start building your stove, take a strip of cardboard the right depth for your tin, and cut a length of it, which will just fit in the tin when rolled up tightly. Cut a short length of cotton string, to be the wick, just a little longer. Place your wick at the end of the strip of cardboard, then roll it up so the wick is in the middle, then place the rolled-up cardboard into your tin.
Set up a 'double-boiler' - a lower larger saucepan filled with hot water, with a smaller clean saucepan balanced inside it. Put the wax into the inner saucepan to melt it. This can be the wax from old candles, or craft/paraffin wax. Watch it carefully until it melts. Once it is fully liquid, carefully pour it slowly into your cardboard-filled tin, allowing time for the wax to soak right down to the bottom of the tin as you pour. Once it is filled with wax to the brim, make sure your wick is lifted up, so it can stick up above the tin, available to light.
Once the wax has set, the stove is ready for use, and can either be used straight away, or be stored until it is needed. Although it can be used as it is, one refinement I use is to take three or four long galvanized nails, and insert them in the wax, round the edges, so that they sit 3cm or more proud of the wax - this allows you to sit another can on the stove as a 'saucepan', allowing you to cook on a one-person scale, to heat water for hot drinks, or mini one-pot meals.
Or you can take a larger can, cut a flap in the open end, and sit it over the burner, in order to use as a grill or frying surface.
A related Guide activity is 'tin can cooking'. In this case, an empty can is used upside down. Air holes are made around the sides of the can using a can opener - an old fashioned type which makes triangle-shaped holes is especially good for this, but you can also make a vent using metalwork tin snips, or make holes with a hammer and large nail.
Place a buddy burner or a household-type candle under the tin can, and use the base of the can as if it were a frying pan. On this you can cook pieces of bacon, sausage or potato scone, pancakes/dropped scones, or bantam eggs - the larger the tin, the larger the pieces of food you can cook on it.