Flip the skinny triangles, the left side triangle will be sewn to 1, bottom to 2, right side to 3 and top to 4. Match the square end of the triangle with the point of the large triangle and begin sewing, gently curve the skinny triangle to match the edge of the large triangle. This is such a gentle curve that it doesn't need to be pinned. It just fits together.
Finger press the skinny triangle toward the large triangle. The directions in the magazine called the large triangle A, corner triangle B, and skinny triangle C.
Lay them back in order.
Shown sewing 4 and 3.
Should look like this.
This backward S curve is the best way to seam it. I tried a C curve and it did not sew together as nice.
Match the cut off corner of the skinny triangle C to the inside corner of the the corner triangle B. Begin sewing and curve the right side to match the left side the center seams will butt together. Pin if you like but after sewing a couple you won't need to pin.