Basically, a Streak of Lightning quilt has overall up & down zigzags. They can be achieved in many ways -- log cabins, flying geese, even squares set on point. Our inspiration quilt is made with 9-patch blocks made up of half-square triangles (HSTs, from here on).
I had been quilting for more than 10 years before I learned the "magic number" you need to add for cutting when you know what size you want finished squares to be. This is probably taught in Quilting 101, but, being largely self-taught, I didn't know it. If you want your HST to finish at 4 inches, you add 0.875 (or 7/8) inch: so a 4.875 inch square, cut in half diagonally & sewn to another triangle, will give you an unfinished square of 4.5 inches, a finished square of 4 inches. (I usually cut a full inch larger, since cutting at the 7/8 line isn't the easiest. Then I trim the block down to the unfinished size.)
For this project though, I wanted to keep the cutting simple, so I am making easy cuts: the Laura Ashley dresses I cut at 4.5 inches. Let's see, that means they will finish to 3.625 (3 5/8) inches. As a separate (& experimental) project, I cut my Elvis fabrics to 3.5 inch squares. They will finish to 2.625 (2 5/8) inches. (Kind of small, I think.)
Sujata has written an excellent post about how to assemble & lay out the blocks. The interesting thing about this block is its asymmetry -- set it in one way & you can continue the zigzag, either in fabric print or fabric value, flip it around & you bring the zigzags to a halt. So much fun!
As for making your HSTs -- whatever is familiar is probably best. There are lots of ways for mass-producing HSTs, but I prefer old-school: two right triangles that I sew together along the hypotenuse. Then, because I love super flat patchwork, I press the seams open. In this case, this is useful to reduce bulk where all those seams are coming together.
And how many blocks should you make? I don't know! I don't know how many I am going to make! I don't know how big my quilt will be! I'm thinking biggish -- double bed or maybe even queen size. But I'm pretty sure I will have blocks & cut fabric left over. The very good news is that HMQG has decided that our fall charity will be to make wheelchair-sized lap robes to donate to nursing homes. These blocks, backed with fleece or flannel, will make cozy & cheery lap covers for some seniors -- a project near to my heart!