i just built one last week.
it's fairly easy. just get a resistor by calculating it at: http://scriptasylum.com/rc_speed/nospark.html
for me, i just went to radio shack and bought a 100 ohm 1 watt resistor for a few bucks (came in a pack of 2) and it seems to work fine. i wait about 2-3 seconds and it doesn't spark anymore.
you just want to split up your pos(+) coming from your battery into 2 red wires. your red battery wire from the controller needs to also be split into 2 red wires. 1 wire will have the precharge resistor between it and the controller. so when this line is plugged in it will look like:
battery -> resistor -> controller
then you have a second red wire that you plug into the controller after waiting a few seconds...
battery -> controller (no spark!)
i'm not sure if the attached image will help since it has more things in it. you can ignore the whole key/ignition thing and just assume that is one wire connected. i also used a hub to simplify the Y splitter / connectors a bit.