Oooh, i know what you mean...how you lay a square on top of a strip or rectangle, then sew corner to corner and then trim? I have found if i trim first, and then press, everything comes out wonky, the seam seams to stretch easier after it's trimmed. Sooo, what i do, is i sew, then Press, then trim. And when i press, i can seee if my triangle is matching up with the bottom fabric, and if it isn't, i press and shape it 'til it is, then trim, and it works really good this way.