Which edge is your armhole edge? It's probably the same as the one you BO 6 sts on. So BO, finish the row (R 1), work the next row (R2), then dec at the beg of the next row (R3), finish the row, do the next one, and dec at the beg of the one after that (R5). Then do it on rows 7, 9, 11, etc until you've decreased 9 times.
sue- knitting heretic