When knitting pattern in the round it is easier to continue all the way up, inc sts each side where armhole starts and then cut open afterward.
Work steeks for e.g. armholes as follows;
inc 5 sts on each side by marker (these 5 sts are for cutting up later and should not be knitted into the pattern).
After bind off sew a marking thread down the middle of inc sts.
Sew two seams on sewing machine each side of marking thread. Remove marking thread and cut between seams.