I don't think you mean flash and clear at the same time, flashing, is like updating the bios, but clearing the CMOS is easy:
Just look...
a) in your motherboard manual
b) manufactures website for the motherboard manual
b) near the cmos battery on your motherboard
For a little jumper, which has three pins sticking outward, and a jumper linking the two.
Once you've found it, make sure your computer is off, then change the position of the jumper (the only other position possable) and hold the power button for 5 seconds, that will clear and reset your cmos.