Above is in a shed so 100% accuracy isnt necessary. Only have any ideas on how to do this, my helpful blocklayers are shrugging their shoulders (young fella's so I'm not going too hard on them) !
AFAIK let the cill protrude beyond the external face of the wall by 3" then fill the gap between the back of the cill & internal face of the wall with concrete mix - make sure they don't fill flush up to the top on the cill so the window cant be seated on it!