You will need a total of 4 pieces of leather in 2 different thicknesses (these measurements fit iPhone 4 and iPhone 4S):
The large outside piece: 285mm by 87mm at around 2.5-3mm thickness.
The inside piece that holds the phone: 130mm by 87mm at around 2.5-3mm thickness.
The two credit card slots made from thinner leather: 97mm by 87mm and 77mm by 87mm at around 1.5mm thickness.
You can use a scalpel or a head knife for cutting. Make sure you use the sharpest tools for nice & clean cuts.