Place two of the triangles right sides together.
Take one side and push the fabric back or forth until the stripes line up precisely.
This will cause a slight gap/overlap on the opposite side, but that is okay. This will be on the outside of the block and will get trimmed away when you square up the block. (This pic shows the other side, the side you are going to sew on should be flush)
Pin your block on the edge where you lined up the stripes. Be sure not to pin with the stripe, your pins won't be straight that way!! I do this all the time, so if you find you do, just move them. LOL
Sew the triangles together.
Do the same for the other two triangles and go ahead and chain stitch (run them through the machine without cutting the thread).
Once they are both sewn, lay them on your ironing board facing the exact same way.
Iron the seams to set them.
Then iron the block open with the seams to one side. I personally would not iron the seams open rather than to one side at this point as it will make your life easier to have those interlocking seams at the next step.
Iron the next side open the same way.
Now you have both sides open.
Place these two right sides together.
You will see that your seams nest together, make sure they are right up against each other.
Now here comes the fiddly part. Once again we want to be sure our stripes line up. This may mean that your top edge is slightly off set. You will always keep the middle seam nestled, but slide the top block up and down along that "spine" slightly until your stripes are all lined up. You can see in this image that my blocks are offset by less than 1/8 inch, but my stripes all line up.
Pin again, and sew. You will sew a quarter inch from the shortest edge, which is why it's good to have a visual on the top of your foot for where that quarter inch is. Remember though if you are off a bit on your quarter inch, that is perfectly fine, they will be squared up at the end.
Set your seam with an iron.
This seam can be ironed open or to one side, I always do one side anyway, but it won't make a difference at this point.
Now stand back and enjoy your handy work.
See how those lovely stripes line up?
Square up your blocks utilizing the 45 degree line on a ruler. (No pics of this sorry!)
I hope this tutorial is useful to someone. As I said if there are any questions, do not hesitate to ask!!
Cheers!
Rachel