Wash the plums and put them into a kettle and cover with water. Cook slowly until plums are soft. Remove from stove, rub them through a sieve and measure the pulp. Add 1/2 of 2/3 cup of sugar per cup of pulp. Cook mixture slowly until thick and clear. Pour into hot jars and seal. Process in boiling water bath canner (212 degrees) for 5 minutes.