You may have seen my "West Elm" Table Mat which I had a couple comments and a question about the Y - Seam, so I thought I'd share my response with everyone!
Just a disclaimer, my Y Seam is probably not perfect, but it turned out pretty good. I looked up some tutorials on how to do it and will add a couple of my own tips. I know that everyone learns differently, so I will share a couple versions on how to complete it successfully.
My favorite tutorial would be on Fresh Lemons . She gives lots of picture diagrams and hers is the most similar to how I do my Y seams. My only adjustment to her tutorial would be at the end of her step 3 where she has a line and dot drawn digitally. I would take a pencil and actually make a dot (where she has a dot shown) on your fabric. The dot should be marked right where the seam (fabrics meet) is underneath so that you can easily see where to stop sewing/land the needle at the end.
If you prefer to see and hear someone explain the steps slightly different. Go to: http://www.youtube.com/watch?v=XLkHfcuNzCA .
Hope this is helpful!
Sweet & Simple (hopefully)!