The first thing you need to do is measure your tiles/flags. The dimensions of your rectangular tiles will determine the options you have.
To measure your tile/flag, place the start of your tape measure along the very edge of your tile, make sure you are not in the grout. If your tile/flag has a slight curve around the edge, do not include this in your measurements, move your tape measure to where the tile is flat and level.
Even a 1 mm or 0.1 inches can make all the difference when stenciling - so be a ACCURATE as you can.