Hi Belle Taine: The best instructions I have 'seen' for Y-seams were on HGTV's "Simply Quilts" but that's been off for several years. I just tried their site to see if I could find those instructions and came up empty, but I don't research very well. Alex Anderson has an e-program now that you may want to try. Also, you may want to look for instructions of blocks that use Y-seams such as "Attic Windows". I'll keep looking.
Hope you find some help.