Ok I think I *might* understand what the problem is. Jennifer's illustration is a starting point. When you sew the triangle to your strip set, the corners of the triangle will
not match the corners of the strip set. They will hang off a bit. Here's a picture - the red corner peeking represents your triangle. You should stitch right where the pin is, which is 1/4" from the raw edge. When you open your seam up, the raw edges of both patches should be even.
Hope this helps!