Out of all of the connectors out there the Traxxas ones are the easiest!
The main concept to them is:
1) presolder the tab up to the marked line
2) pretin the wire by applying solder onto it first
3) heat the tab and the wire up at the same time.
4) slide the tab into the connector
Place the tabs on a piece of wood (something that is flat and that can take the heat) with the marked lines face up.
Place solder onto the tip of your iron and place the iron on the tab. The heat of a 40+ watt iron will heat the tab and transfer the solder onto the tab. Apply more solder if need be.
Take your wire and trim the wire down so that the length of the exposed wire is not longer than 4mm.
Next you will want to pretin the wire. To do this, apply solder to the tip of your iron and put the iron on one side of the wire. Put solder on the other end. When the wire heats up the solder will start to melt. Paint the wire so that it is covered in a light coat of solder.
With both parts pre-tinned you will want to solder the wire onto the tab. I personally will heat the wire up at the top first and then slide the iron to the side so that it touches both the tab and the wire. If you prefill the iron then you will not need to apply more solder.
Slide the tab into the connector.
And with that.. you are done!