The easy way is exactly as you have said using timber
depending on the width of the cill I put 2 or 3 pieces of say 25mm thick
timber batten upright below the cill and fix these to the brick wall with
pins or screws
then nail another piece of 25mm thick timber horizontally across at the top
to support the outer edge of the tile. (it doesnt reallt need supporting but
what you have now is a guide edge approx 50mm out from the wall.
The bed your lower tile on normal mortar usinfg the front edge of the tile
onto the batten. Bed your top tile at mor of a slope than the bottom one and
there it is
Allow to set for a day or two and remove the battens, Using the uprights
means you can point/tidy the joint under the tile