another tip, which may or may not help you, is using denatured alcohol before wiping your bead. i would use the aforementioned suggestion of using masking tape to lay out your seams. so once you have your tape in place, then go ahead and apply your silicone bead. at this point there is two ways of going about cleaning your bead, you can use a tool or just use your finger. i use my finger but i dont think that wil give you a wide enough bead... anyways, after applying your silicone bead, spray it down with a mist of denatured alcohol before you smooth out your bead. the alcohol will kepp the silicone from sticking to anything that its not already stuck to. not only that but it leaves the bead very smooth and clean looking.
thats my metohd for perfect $20,000.00 stone to stone silicone beads on granite counter tops
good luck!