Cutting... Best time June/July, cutting of firm new growth. apply 0.3% IBA (rooting hormone) stick in well drained soil & mist. takes 40-80 days to root.

Seeds: mash berries and remove seeds. Mixing the mash with water and "panning for gold" is the easiest way that I've found to sort the seeds, which will fall to the bottom.
If I remember, winterberries require two freeze/thaw cycles to germinate, and so will not sprout until after the second winter after planting. Even then, germination is low. The easiest way is to buy seedlings.

Here is a link that might be useful: winterberry seedlings