Rotary tool with a diamond tip that fits in your marker and a cutting disc
Unassembled USB (the smaller the better)
Unassembled marker (with all the parts of the body) You should use your pliers to help disassemble
A pair of pliers
LOTS of Ingenuity
Use your Rotary tool with the cutting disc and cut off the knob where the ink nib used to be. Then use the rotary tool to sand out the inside of the main body of the marker using the diamond tip until the body of the USB (the end you do not stick in the computer)fits in the main body of the marker. Sand out the inside of the cap so that the actual USB fits in it. Reassemble the marker with USB inside. Have lots of spyish fun!