Pit the greengage plums, and then freeze them overnight.  This breaks their cells to help them release juices but also helps damage the pectin that would cause a cloudy wine.  

Defrost the plums in a brewing bucket, and pour 3 quarts of boiling water over them.  When completely cooled, add pectic enzyme, yeast nutrient, lemon juice, and wine yeast (but not the sugar).  Cover with a towel or loosely with a lid and allow the mixture to ferment for 4-5 days.

Dissolve the sugar in a quart of water on the stove, stirring to ensure all the sugar is completely incorporated.  Allow the syrup to cool.

Strain out the fruit mash through cheesecloth, collecting the juice.  Place the juice into a sterilized carboy, and top with the sugar syrup.  Add more water to fill if necessary.

Seal with a water lock and allow the mixture to ferment at room temperature for 2 months.  When fermentation has stopped, and no bubbles move through the airlock for at least 5 minutes, it's time to bottle.

Bottle the wine in sterilized wine bottles, cork, and bottle age for at least 6 months before drinking.