Then you lay the piece to be sewn on top of the star, right sides together with the edges parallel, and the intersection firmly in line with the seam line. You can poke a pin throught the intersection to check it's directly on the seam line if you like. Put a pin in to secure it while you sew.
Sew along the seam line up to the intersection and stop with your needle down.
This is the only tricky bit. Swivel the pieces round, so the next two edges are ready to sew. Try to straighten both pieces of fabric, especially at the Y bit, so they lie flat (or at least flattish!). Then continue sewing right to the end of the shape.
It should look something like this when it's finished.Now press and hey presto, a flat Y seam!
OK, it would have been better if I hadn't lost one of my points, but a bit of fudging will solve that. At least my Y seams are tuck and lump free!