Here are directions for sewing your binding strips together using a diagonal seam along with directions about how to attach your binding to your quilt. I usually cut my binding using a 2 1/2" width, but this will work with any width you choose.
Begin by trimming all the binding strips so the ends are square so they all look like this.
Lay them out to sew together as follows:
1) the first strip (black) is Right Side Up and laying horizontally
2) the strip to be added (purple) is Right Side Down and lined up at the corners (like the photo).
Make a chalk line that runs diagonally from the left top edge to the right bottom corner (the bottom corner of the black fabric , but you can’t see it because it’s under the purple). Sew the pieces together on the marked line.
Take the other (free) end of the piece just added (the purple), flip it so that it is Right Side Up, bring it to the machine, and add the next piece in the same fashion.
If you just keep adding new sections of binding without pulling it all away from the machine, then all the pieces are lined up close together and ready to trim and separate.
Trim the seam allowances, iron the diagonal seams open and iron the binding Wrong Side Together the length of the binding. Attach to the quilt as usual. On this project I’m attaching it to the back of the quilt, because I am going to finish it by machine on the front, but the steps are the same as when you attach it in the typical way to the front. When you have sewn the binding on all around the quilt, leave about 12″ open when you reach where you began. Leave tails for trimming in the next step.
The tails need to overlap the measurement that is the width of your binding. I cut this binding to 2 1/2″ so the blue fabric needs to be trimmed so that if only overlaps the black fabric for 2 1/2″.
It should look like this. That’s my 2 1/2″ ruler.
Now undo the fold in the two bindings and position so that they are both laying Right Side Up.
Position the two fabrics to join together just the same way you joined the binding strips together when you prepared the binding strips. The black strip is Right Side UP and the blue strip is Right Side DOWN. You’ll have to bunch up the quilt to position them this way. If you left an opening that is too small, it is really hard to position to two fabrics like this!!
Mark the diagonal line from corner to corner just as you did before, and sew on the marked line.
Check the seam that you have just sewn by laying the binding on the quilt and checking that everything is looking right. When I first learned this technique, I would sometimes twist the binding without noticing it. If everything is looking good, trim the seam allowance and finger press open. Lay the binding strip flat and sew the remainder of the seam.
Here you can see the binding sewn to the front. This ending seam looks just like all the other seams on the binding.