Some people don't pre-cut the strips to length first, they use a cut as you go method. Since you are just making one block, I would do this. Here's a link on the how-to:
http://www.marthastewart.com/907228/...-pattern-quilt
If you feel more comfortable, make the block up to the 12 inch size (12.5" is the measurement including the seam allowances), then use the method above to add the rest of the strips. You can always just oversize the block and trim it down to the size you need by centering the block and trimming the outside row evenly.