Take the metal connector piece and a pr of pliers (needle nose if you have them) and use a rubber band on the plier handle to hold the piece of metal with the end to solder pointing out. Heat the piece of metal - hold solder iron under it til you melt a little solder onto the metal where wire will go (this is called tinning). Take the wire, strip insulation back a little (~1/4") and twist the strands together - I use a vise to hold the wire and heat it with the solder iron as I hold solder on the wire, when it gets hot enough, the wire will pull the solder in and it will be tinned.

Take the wire and hold it on the metal connector end where its been tinned and heat under the metal. Hold it until you see the solder melt on the metal piece and on the wire. Without moving the wire, take heat away and wait for it to cool and solidify - will be a good solder joint. Takes longer to read than it does to do it. Prob is holding everything, you need 3 or 4 hands. The pliers and a vise make it easier.

Here's traxxas' how-to on soldering their connection - 2nd one -

http://www.traxxas.com/products/video/support/