cone wrenches are pretty much necessary to make it perfect, and they're cheap.
but here's the trick to adjusting cones on a coaster brake:
make sure the sprocket side cone nuts are tightened together.
then, on the brake arm side, tighten the inner cone nut down by hand till you can feel the bearings grabbing/dragging, like the nut's too tight. put the arm on.
then run the outer nut down against the arm by hand also.
now, put the arm at the 7 o-clock position and put a wrench on the outer nut at the 4ish o clock position, and push them together to tighten them up.
9 times out of ten, the arm will move and not the nut, so it's loosening up the drag on the bearings.
now, hold the wheel down and lift up and down on the brake arm, and see if there's any movement. if there is, it's too loose. repeat steps 1 though whatever above and try again.
if it feels alright, pick up the wheel with a hand on each axle and spin it. if the axle turns in your hands with the wheel it's too tight.
set the wheel back down, put a wrench on the outer nut on the sprocket side, and lightly beat on the coaster brake arm in an anti-clockwise direction with a hammer. (i said lightly!)
hopefully, it'll move just enough to adjust it.
if not, take it to a freakin bike shop. let them deal with it.