You can either solder the wires directly to each other, use power poles, or larger connectors. If you decide to solder the wires directly, temporarily hook the wires together to make sure the motor spins in the right direction before soldering for good.
For direct soldering, I find
these to work well. Just cut off the plastic housing and solder the wires into the metal part (don't crimp). Once heatshrunk, it makes a nice clean inline connection.