Nearly all the rules for jam-making apply in marmalade-making. The fruit is first simmered gently, usually in an open pan, until it is thoroughly softened. After this, the sugar is added and stirred over a gentle heat till dissolved. Then the marmalade is boiled rapidly, with a full, rolling boil, until setting point is reached.
These are the essential differences: (a) The peel of citrus fruit takes longer to soften than the fruit used for jams.
(b) Because most of the pectin is present in the pips and the pith —rather than in the fruit pulp or fruit juice—these are important ingredients of marmalade recipes. The pips and pith should not be discarded (unless they are being replaced by commercial pectin) but should be tied loosely in muslin and cooked with the fruit until the pectin has been extracted. If the muslin bag is tied to the handle of the pan, it can easily be removed before adding the sugar.