To make a 12 1/2" block you need to cut your material 4 1/2".
This will be unfinished block and when sewn with the rest of your blocks will than measure 12". You should use an accurate quarter inch seam. Take a ruler with a quarter inch measurement place it under your needle in your machine and mark where the quarter inch is from the needle. You can either use a quarter inch foot that you can buy genetic which should be cheaper than your normal quarter inch foot for your machine. If you are limited in funds just place a tape like masking tape for quarter inch measurement.
Remember usually the 12 1/2" block goes together as a nine patch.