Kerry has given some really great info in Version 1 on how to spot Y-seams in a finished block and how to do Y-seams using freezer paper, so make sure and check her version out as well!
What you will need:
Your fabrics, the pattern (which has the side Y-seam portions cut off the spool), pencil, rotary cutter, mat, and a small clear ruler.
First, foundation piece the spool. When you sew this part, start and end sewing exactly where the line on the pattern starts and ends.
Once you have the spool pieced on both ends, use your ruler and a pencil and draw a line 1/4" out from the edges of the spool.
then use your rotary cutter to cut the spool out along the penciled line.
Trace your side template pieces onto fabric
Pin in place if you wish (I didn't) and flip the piece over and sew just to the side of the paper pattern from the top stitch line to the bottom stitch line. Repeat on the other side.
Now flip the piece over and pin the two edges together on one corner, indicated by the black arrow. The red arrows indicate which edges you are pinning together. When pinned, you can see that the piece is pulled up on that corner.
Starting sewing at the outside edge corner until you reach the stitching corner indicated by the blue arrows. Stop sewing exactly where the the side stitches stop. Repeat for the other three corners.
Press your side seams open on the back, and you are done!