heating pad circuit.png

Proto-Pic Heating Pad PPCOM-11289

Check the pins of your MOSFET transistor are arranged in the same way shown - they may vary. Google the part number printed on the transistor.
A good video explanation of MOSFETs and the pin namings is here.

You can run the heating pad off a 9V PP3 style battery (the squarish ones) but it probably won't last more than an hour.
the best technical instruction is with the Pic16f877a