Q:

How can I create a DC-DC voltage multiplier

I want to take the 5V output from a USB port and make it 12V, so that I can run a cooling fan which is 12V. Is there any circuit I can make at home myself?  If the output voltage is not exactly 12V, it can be at least 11V so the fan can run smoothly. 

A:

You could use an MC34063 boost converter circuit to do this cheaply.  

The parts values will be different, but the circuit will be the same.  Calculate the parts values using the equations in the datasheet page 11.  If the port can supply 500mA without complaint, then you could get about 100mA @ 12V into the fan assuming 50% efficiency.  USB only guarantees 100mA without negotiation. 