You're always dec at both ends...meaning each edge (as pattern states: 'at each end'). By doing them in 'pairs' your armholes should be the same.
Your BOs will be done on diff sides but then the decs are worked on the same side each time. Work a dec...work across...do a dec. Turn, work across...no decs. Repeat the dec row.
RS: Dec at beginning, dec at end for 7 rows.
RS: Dec beginning, work, dec at end
WS: No decs
Rep last two rows.