Here is my method for assembling a Y-seam unit. (For this tutorial, I will refer to the pieces by their color and shape.)
I’ve included lots of photos and many instructions so that you can see and follow what I do and why. (I just don’t want you to think, “Is this what she means?” or “Is this the way it’s supposed to look?”) It may seem like a lot of steps for one unit, but, relax, after you’ve sewn a few, you’ll see how easy it really is and that you can do it!
After cutting out all of the pieces, mark each piece, on the wrong side, 1/4″ in from both sides at each corner.
Lay out the pieces, in their respective positions, wrong side down.
Flip the top triangle down over the left diamond. Align the raw edges.
(There will be a tiny overlap of the triangle piece.)
Align the pieces by inserting a pin into the intersecting mark at the top, left, corner of the beige triangle, then into the dark blue diamond so that it comes out at the intersecting mark on the opposite side of the diamond. Leave this pin in place for now. Insert another pin, at an angle, beside the first one to keep the pieces aligned.
Insert a pin into the intersecting mark at the vertex of the triangle (The corner opposite the long edge.) and into the corner mark of the dark blue diamond. Leave this pin in place for now. Insert another pin, at an angle, beside this aligning pin. If you wish, insert a pin in the middle of the two “holding” pins. (Sometimes fabric pieces can shift when pins are inserted. These “aligning” pins just act as a visual aide to assure that there has been no shifting when the other pins were inserted.)
When the pieces are pinned and ready to be sewn, remove the two aligning pins from the intersections. Insert the top, left corner of the unit, with the beige triangle on top, under the presser foot. Put your needle down, off the edge of the fabric. Sew to the intersecting mark. (I have found that my seams are more accurate when I sew to an intersection, rather than backstitching toward it and trying to land the needle exactly on the intersection.)
Lift the presser foot and rotate the unit 180 degrees so that the seam is now facing you. Sew 3 to 4 stitches to lock the seam. (I turn the unit so that I can see exactly where my needle is headed. Sometimes when I try to backstitch to secure the seam, I find the needle has drifted to one side of the seam or the other. This step insures flat seams when pressed.)
Do not press the seam at this time. Lay the unit right side up and align the medium blue diamond in position.
Flip the medium blue diamond up and onto the pieced unit. Align the raw edges.
Insert a pin into the intersecting mark at the right corner of the diamond, and into mark at the top right corner of the beige triangle. Place a pin beside it as before. (Not pictured.)
Rotate the unit so that the left (dark blue) diamond is at the top, as shown. Insert a pin into the corner intersection of the medium blue diamond.
Fold back the seam allowance of the pieced section toward the dark blue diamond. Insert the pin into the beige triangle at the mark.
Place a pin beside this aligning pin. (I usually place the pin in the seam allowance to keep it out of the way as I sew.) Sew from the raw edge of the diamond tip to the corner mark. Turn and sew 3 or 4 stiches to lock the seam.
Open the unit and lay it right side up. Rotate it so that the unsewn seam between the diamonds is at the top.
Fold it in half and pull the triangle fabric back out of the way..
Insert a pin in the marks at the tips of the diamonds and pin beside it. Pull slightly on the beige triangle to pull the seam allowances out of the way. Insert a pin into the marks at the diamond corners. Be careful to not catch the beige triangle fabric with the pin. insert a pin to hold these seam allowances out of the way.
Sew from the edge of the diamond tips to the mark. Rotate and stich as before to lock the seam. Now, it’s time to press.
I like to finger-press the center seam open. Or use my Unusual Pressing Tool to get it flattened before pressing.
Then press the triangle seams toward the diamonds. This just helps keep the unit nice and flat.
I’ve tried pressing every seam open, but then the folds that are created at the center of the “Y” get tricky to lay flat.
I do not trim the “dog ears” or “Swallow tail” until the unit is sewn into a block. These little bits can actually help when lining up to other units.
As always, if you have any question about this, or any other tutorial, email me at firstname.lastname@example.org and I’ll be happy to help.