Put guavas into a saucepan big enough to hold them all and leaving a good 5 - 10 cm space above the level of the guavas. No need to peel or cut them.
Add enough cold water into the saucepan to cover the guavas.
Boil the guavas for an hour. They should have become very soft and some will have fallen apart.
Put the colander over a bowl big enough to take all the liquid in the saucepan.
Line the colander with enough muslin to drape over the edge.
Pour in the guavas and liquid from the saucepan. Let the liquid drain out. Don't press the flesh of the guavas through the muslin. Just let the liquid drip through. Leave the liquid to drain for an hour.
When all the liquid has drained discard the muslin and the guava pulp. You can wash and re-use the muslin.
Measure the liquid with a measuring jug.
Return the liquid to the saucepan.
Add 1 kg caster sugar for every 1 litre of liquid to the saucepan.
Add the juice of half a lime for every 1 litre of liquid to the saucepan.
Boil the liquid fast. Keep watch over it and as scum gathers on top of the liquid, scoop it out with a soup lade or spoon, Don't worry if you take some of the liquid with you as you do this. I find it useful to have a small bowl to put the scum into.
Boil the liquid until setting consistency is reached. To test for this: take a teaspoon of the liquid, and put it onto a small plate. Put the plate into the refrigerator or somewhere cool. After five minutes take the plate out and tip it on its side. If the liquid doesn't run down the plate the jelly has set. You may need to do this a number of times during boiling the liquid. The time to get to this stage depends on how much liquid and sugar you have used
While the liquid is boiling, put your glass jars on a a tray in the oven heated to 110C. This is to sterilise the jars.
When the jelly has set take the saucepan off the fire.
Take the jars out of the oven.
Fill the jars, fit the top on and that's all there is to it.