Someone may have discovered this before me, but here is what I have discovered while lining up corners when assembling strips of diamonds for these quilts.
First, find the little points like the one the pencil is pointing at in the photo on the side of your strip of diamonds that you want to sew to the other strip.
Next find the edge of the seam line on the second strip of diamonds as shown in the photo below.
Next, line up the point on the top strip to the edge of the seam line on the bottom strip and sew. If you are precise, your corners will come out perfectly every time and this saves a lot of time marking and and sticking pins through etc.
The only time during this process that this method is not usable is when you’re joining the two sets of strips needed for a six-color star (see photo below), because neither of the strips you’re joining has the little point, so you have to do it the old-fashioned way.
I really love Eleanor Burn’s book on Radiant Stars. She makes this complicated design easier than other methods because it’s mostly just sewing strips together.