Just in case you are still curious what your issue was: the outside of the square should be 10 pixels wide, but yours is only 8 pixel wide (you cut from 1 to 10 leaving only pixels 2, 3, …, 9 - 8 in total). Same for the inner cut, which should be 4 wide but you cut 6.
The result preview also highlights this, the black outline is still part of the solution and it got a red tint, means you cut to much.
I guess a tutorial would help, but in general it should be a challenge to get right and you have to perform some arithmetic in your head ;)