It will be impossible to match the old and new, so depending on the scale of the job, it may be woth considering removing all the old dashing to a natural break, i.e. a corner.
whichever way you go, you will need to remove all the blown render back to a firm base.
You will need to glue the birickwork, and apply a scratch coat base. After that, apply a top coat, and while just tacky (you will have to practice this), you flick the stone onto the topcoat. All the excess falls onto the floor, so make certain you shhet the ground below where you're working.
This can be messy, and to be honest, if not doen properly can look absolutely awful. Maybe a call to a plasterer??