Remember those snowball corners I made the other day? I've been joining them onto 9-patch blocks. When I do this, I'm joining an angled seam to a straight seam so it is a little more difficult to make them match accurately. There is a simple trick though. Use a pin to match the points of the seam, 1/4" down from the raw edge. Be sure you do not measure 1/4" along the angled seam as this will not be accurate - it needs to be measured straight in from the raw edge.
My close up photo shows where the pin goes through both sections. The angled seam is on top and the straight seam you can see behind. The pin is going through both seams exactly where the next seam will be sewn (1/4" from the edge).
This photo shows where you stitch the seam, you can see the pin pierces the fabric exactly in line where the seam will be sewn (seam line is indicated with dotted line). When stitching the seam, sew right up to the pin before removing it.
After the seam is sewn, press to one side and open it up to see perfect points where the red fabrics meet.
Give it a try!