OK, so here is a quick visual which will, hopefully, make this simpler for you.
Your first triangle has 12 stitches:
You then turn your work, inc., SSK:
You will notice that there are still only 12 stitches, total, on both needles - you have not increased your stitch count because you inc. on the first stitch but then you SSK to dec.
Carrying on in this way across the first triangle you will then be creating your next triangle...
Does that help at all?