12 inch blocks are great for baby quilts because 3 across and 4 down, makes a 36 by 48 size quilt, or 3 across and 3 down makes a 36 inch square quilt. Both are good sizes for baby or toddler quilts.
I wanted to make a twelve inch block like this one in my old "Lap Quilting" book by Georgia Bonesteel. The blocks in this book all use templates and I still have my cardboard templates that I made from instructions in this book in the 80s and 90s. But, I definitely am only going to use my rotary cutter nowadays! So what I did was to measure one side of the triangle template in the book, double it and cut squares of that size.
The size of the square needed for the large triangle is 6 7/8 inches. The size of the square needed for the small triangles is 5 1/8 inches. Then slice the squares in half like this.
Then sew the triangles together. Here I am chain piecing the smaller triangles.
Here is what they look like after being opened and pressed.
You will then sew the larger triangles and the pieced triangles together to make 1/4 of the block. In this picture I am using the leaders and enders system written about by Bonnie at Quiltville. You can see some purple four patches that will be part of my rainbow double Irish chain quilt. This saves thread and you are making progress on a second quilt at the same time.
You can see the 4 separate sections of the block here.
Another hint that helps your blocks to lay flat is to put a stack of books on top of a freshly ironed and still hot block. There's a block under this stack of books.
I tried different methods of pressing the seams on these blocks to see which one helped the blocks lay the flattest. I have to say that opening the seams worked the best, but was too time consuming. I ended up pressing them toward the dark and spinning the seam on the center of the blocks.
I hope this has been helpful!