Sort out and discard all the scraggy and small feijoas. Discard any that are very ripe - a few hard green ones help it set.
Rinse loose dirt off fruit collected from the ground. Cut off heads and tails, and slice fruit in half length-wise.
Place fruit in saucepan, skin and all, with not too much water - just enough to get the fruit steaming.
Bring to the boil and simmer until soft - about 5 minutes (they turn an ugly green colour).
Strain through a jelly bag or very fine muslin/linen overnight. Do not squeeze the bag. The juice is a greyish green colour - do not worry.
Add one cup of sugar to every cup of juice collected. Bring sugar and juice to the boil. Boil vigorously for approximately 10 minutes, stirring occasionally.
Add a knob of butter to reduce froth. If the jelly does not set, keep boiling until it does. Test for setting (drop a few drops of jelly on to a saucer which you have cooled in the freezer).
Pour into pre-heated jars. Allow to cool and seal.