Plums usually 3-4lb per gallon, so you'd be looking at 15-20lb of damsons.
Halve (and destone them, freeze them, defrost them) put them in a bucket with cold water and pectolase and 1 crushed campden tablet per gallon of intended wine (5 in your case). Cover and leave over night.
Next day (or day after that) SCRUB your hands thoroughly with a nail brush and preferably antibacterial liquid soap. Then get stuck in, put your hands in and squish everything thoroughly through your fingers.
Pitch your yeast and leave for a matter of days. I'm not normally too precise about how long, because it depends on when I've got a free day. But 5 days is about normal, I think.
Strain through muslin/straining bag into a DJ or other fermenting vessel and add sugar*.
Ferment to dry, back sweeten to taste.