You had better find preferably a west of Ireland plasterer to ask him advice or do it for you. I say west of Ireland because just as in the UK the style of houses changes with the area.
From what I remember seeing someone for a short while doing it there are strips of timber hammered to the wall to either support or position these corner stones. And the reveals around the windows are off a heavy concrete sill and or formed with lots of timber hammered to the wall and all hand forming.
I think the corner stones are put on before the scratch coat. But sometimes the block work may be 'scudded' first with 2-1 sand and cement............................not sure but plasterer would know.
But to be honest a west of Ireland plaster would fly through this job. Irish plasterers would p1ss English plasterers as in Ireland everything is sand and cement inside and outside on block. Their mixers are big towable diesel mixers not little half bag's.
