Iceblossom

One more edit first: The discussion for setting triangles only happens if you are setting diagonally and with whole alternate squares. If you do the block as designed with the triangles as part of the block, you don't have to read any further! It is a straight horizontal setting and is the easiest way. The fabric I want to use is rather large and I think would work better as a whole alternate square.

Sure, Rhonda, stay stitching is one technique and there are many ways to deal with bias edges. If you use unwashed fabric or starch, that's usually enough to stabilize it but these are rather large lengths. I'd do ok with my crisp ironing (no starch) and letting the machine do the work but some people might warp it a bit just due to the length.

If I wanted the edges straight of grain, I'd cut out that 13.5" block -- I use parchment paper for stuff like this (what you see in the picture), clear tape is better but I have the blue tape handy in my sewing kit. I get the paper at the dollar store for a 1 foot x 25 x roll and use it for many quilting related things!

Anyway, make a 13.5" square (or maybe you have a large enough ruler, I have one but I don't think most quilters do) out of paper taped together and slice it in half along the diagonal. I could look up the math of this, but most of the time I just use paper. That triangle is now your cutting guide, you'd cut a strip the height of that cut piece, and placing the long edge of the triangle on the straight of grain, I think you'd get a total of 5 pieces, 3 on the start (bottom or top) and the 2 in the middle, plus left over trims.

There are sites/calculations on how big a square to cut into an X too -- but these are big and I don't think would cut well without leaving lots of scraps/extra yardage.

If you have a large enough ruler (I have a 16" square) you can mark the 13.5" line with tape, like I did here for this project where I needed a 7.5" cut and the closest ruler was 8.5". I always use the top of the tape as my line, sometimes I need to put in an extra mark on which way is the correct way!