Centering the guide using the wires is simply a matter of creating equal tension on both sides of the guide, tension administered by the guide wires themselves. In some cars, this is not possible, because the guide wires are not directly connected to the guide, but rather some other connection mechanism. SCX cars are a good example of this, as are many older Scalextric cars with their little metal tines that are embedded in the chassis which touch the braid. In those cases, the cars often have their own guide centering mechanism, usually some kind of spring. Where no spring is present, and in some cases even if there is a spring present in addition to wires directly connecting to the guide, it's best to use the wires themselves.
What you do is pull any additional wire back away from the front of the chassis, with only a short amount up front, usually at 90 degree angles, or thereabouts, to the blade. You want enough wire up front to allow the guide to move freely to its fullest extent either direction. If you notice that the guide wants to point one direction, try pulling the wire on the opposite side. Sometimes there's not enough slack to do that and still have full free movement. In that case, you want to give more slack to the side that the guide is pulling toward. It's often necessary, especially in basic cars from Scalextric, to disassemble and manipulate the wires so that they are better arranged to perform this task. In almost every case, you'll need to take the body off of the chassis.
I received another Pioneer Mustang for Father's Day yesterday, and had to remove the chassis, remove the wires from the guide completely, and even remove the front axle assembly. I put the chip in and made sure those wires were arranged to avoid any pinching by the interior, then ran the guide wires up and under the front axle and then to the guide. This created a tensioning loop back for each wire and both sides of the guide. After putting the guide back together and back in the car, I pulled the guide wires back on each side so that the guide would be pulled back to center by the wire on the opposite side of the guide from the direction it was turned.
Slot.it cars come already set up this way and their chassis are designed to channel the wiring in this fashion.