This is how I've been fussy cutting squares, but ask JanieW how she did those lovely triangles in BHeart.
When you cut your fabric square make it bigger than you need by at least a 1/2" making sure the design is centered exactly. Now fold the fabric square in half on the diagonal once and then fold the other diagonal forming a fabric folded X. On the paper square draw diagonal lines from corner to corner. When ready to piece match your folded X to the drawn X on the paper. Pin in place or use double stick tape to keep it where you want it. Glue works, too, but then dries and is harder to take paper off.
Clear as mud?