It is prepared with fruit juice, and you should only use fruit which has enough acidity and pectin (essentially the thing that makes it turn into jelly). I realize that nowadays you can buy all sorts of additives and pectin that will act as firming agents but this is meant for natural jellies without those.
The fruits that contain pectin are sour apples, nearly ripe raspberries and grapes and they are the easiest ones to make jelly with, you can mix them with other fruits too.
It is generally good to add some lemon juice to any fruit when making jelly because of its acidic properties.
If you want your jelly to look really nice don't peel your fruit or take out the core because they contain the most pectin.
Instead wash the fruit and cook it either whole or just cut up, with just enough water to cover it.
Cook it on high heat so it would get soft, but be careful not to have it fall apart.
You should line your strainer with two layers of gauze and leave the fruit in it to drain on its own. Do not press it just let it sit there for a few hours and do its thing.
The juice drained from the fruit should be weight and then added the same amount of sugar, then cooked on low heat.
In general the jelly is done if you put a drop of it on a plate and it hardens straight away.
There is also raw jelly which can be attained from cherries and from red and black currants.
The fruit should be washed and seeds removed then the juice pressed out.
The juice should then be warmed up, but not to the boiling point.
once warmed up turn the heat off an add the same weight in sugar to it.
Keep stirring it well until the whole thing starts to cool and harden.
The raw jelly should be done in small quantities.