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.
yup, this just wants to say: decrease on that side of you piece that your armhole is on. (if the chest part is made in one piece (without deep cut V-neck or anything) then it shlould be on either side. Often I find descriptions like that on patterns that have the deep V-neck or a jacket pattern that has a collar edge as well. Just to tell you, what edge you should work on.