This recipe is originally from here.
The trick to this is using ripe, firm, perfect plums. Inspect everyone for bruises and bad spots before using. This recipe will roughly fill one large 1/2 gallon mason jar double or triple.
Slivovitz (Plum Brandy or Schnapps)
2 1/2 pounds perfect plums
1 1/2 cups sugar
1 3-inch cinnamon stick
2 1-inch pieces lemon peel
4 + cups grain alcohol
Use a sharp knife to pierce the fruit through to the pit, cutting each plum 3 or 4 times. Pack the fruit into a glass jar(s) and add the sugar, cinnamon stick and lemon peel. If in more than one container try to divide equally. Pour enough grain alcohol to cover the plums, and cap the jar securely. Every day for 2 weeks invert the jar or until sugar has completely dissolved. Place the jar in a closet or other dark space for 90 days. Strain contents through cheesecloth and store in a glass container.