A pressing matter
Your block will only be as accurate as your cutting, sewing and pressing is. To help with the last, I trace a square the size of my unfinished block on my ironing board. It helps if you have a plain, muslin style cover! For the 2 1/2" squares sewn together, the unfinished 4-patch should measure 4 1/2", so I draw a 4 1/2" square. This gives me an accurate guide when I press. And yes, I use steam - at least until the iron runs out of water...When a block is particularly unruly, I will try and coax it by securing it with pins before I press.
You'll notice another square drawn, measuring 6 1/2". This is a size often used in quilting . You could easily add a 9 1/2" , a 12 1/2" or any other size square- just remember to add the 1/2" seam allowance to the finished size of the block.
I found a ballpoint pen works better than a Sharpie, which tends to bleed a little.