You can make this wine with stones in or out, the easiest way for me is to leave them in.
Add the plums to a fermenting bin mashing them by hand as you go (just open them up).
This way you can check the plums are healthy (this can be more advantagous if using wild plums).
Add sugar to 1 pint/0.5 litres of water and bring to the boil, add this to the bucket while still very hot.
Add 4 pints/2.25 litres of cold water, stir and wait to cool.
Now add citric acid, tannin and pectic enzyme, stir well cover and leave for 12 hours.
If using activated yeast start your yeast now.
After 12 hours from adding citric acid, tannin and pectic enzyme
Pour your prepared yeast culture (or satchet of yeast) into the must, and add yeast nutrient, leave for 2 days stirring 2 or 3 times a day.
Strain must into a demijohn, it may be easier to remove solids beforehand.
Add the grape concentrate of choice and top up demijohn with cold water.
Fit airlock and move to warm place, for the first few days give it a shake daily.
When fermnetation finishes move to a cool place to settle.
Leave one week then rack if necessary.
When fully clear bottle and maybe sweeten to your taste, then enjoy, this wine can be ready in under 12 months.
If canned fruit is used reduce the sugar content to 1½lb/675 grams. and use 3 x 1lb 425 gram tins.