Are there templates/patterns included with the instructions?
Are you allowing for seam allowances?
When you say you are making an 8-inch block, is that an 8-inch FINISHED block?
If so, then the block-in-progress should be 8 1/2 inches before it is sewn to the other blocks.
If you are attempting to make an 8 inch finished block, the center piece should have been cut 4 1/2 inches square, the outer pieces should be cut 2.5 inches wide - this is to allow for the 1/4 inch seam allowances.
At any rate, good luck with your project.