Generally speaking, it is best to use a flasher when trolling with spoons. Leader length will depend somewhat on where you are fishing. Right now, on the east coast of Vancouver Island (Nanaimo area) whst seems to be working well is a green glow flasher trailing either a black and white ("cop car") spoon or a chrome spoon using a leader length of 48 inches. When fishing on the West Coast (Bamfield, Barclay Sound) I use longer leaders (up to 5 feet - slows the action for the bigger fish) and while spoons work well, I prefer to use bait.
I have had only limited success trailing a spoon without a flasher directly attached, and in that case I used a "dummy flasher". A dummy flasher is a flasher that is trailed about 10 feet behind your cannonball on the downrigger and then you clip your spoon onto the downrigger cable about 6 feet above the flasher and then run the spoon about 10-15 feet back of the cable. so that it is just above and behind the dummy flasher.
Hope this helps.
Dick