I'm not sure how to get this one block trimmed from 12x12 to 10x10 so that it looks right.
What I would do is sit down with paper and pencil and think and measure the different pieces out. After getting the measurements drawn do it again on a different piece of paper. Don't forget that on each piece you have to add 1/4 inch seam allowance all away around each piece. This process can be done if a couple of different methods depending on what works for you.
This is how I would do it. I would keep the 12x12 to use as an extra block sample just for the layout. You can always add the Broken Sash block to the back of your quilt as a surprise.
Keep asking questions here, you'll get many different replies.
Again, don't forget to add in the necessary seam allowances!!
Good luck.
Pam M