The illustration shows a typical Y Seam. The three legs of the seams meet at a central point. Each of these seams are sewn separately and all sewing is begun at the central point.
To sew the first leg of the Y seam begin sewing at the point where the 3 seams will meet, back stitch to lock and then complete the seam as illustrated.
Press the seam open and trim off the tails.
To sew the second leg, position the piece as illustrated and begin sewing where the 3 seams will meet, back stitch to lock and complete the seam.
Rearrange the piece to be able to sew the third leg.
Sew the third leg of the seam, starting where the 3 seam meet, back stitch and complete the seam. Open out the unit and press the 2nd and 3rd seams down towards the diamonds as illustrated.