Hi Cindy,
OK. You want your finished block to be 12 inch.
You want the block pattern to be a 9 patch block.
That's 3 rows of 3 blocks, so each block is 4 inch finished.
So 1 row, 3 blocks, 4 inch each block, 3 x 4 = 12 inch finished
For each section or square of the block you need to add
your seam allowance to all sides, which is generally 1/4 inch.
So your 4 inch finished square is 4 1/2 square to start.
So for whatever size block finished you have, then add 1/4 inch seam allowance at each seam.
It sometimes helps to sit with a piece of paper and draw it out then add the measurements and add the seam allowances at the seams.
Hope this helps.
Pam M