Halve and pit, If you prefer to peel them, it's easier to do this before halving and pitting them. 

Cut the plums into wedges or  halves. 

Lay the cut plums on baking sheets in a single layer. 

Put the trays in a freezer until the plums are frozen through. This will take anywhere from several hours to overnight depending on how thick the plum pieces are.

Transfer the frozen plums to resealable plastic bags or another air-tight container