As was said before don't leave the iron on the motor solder tabs too long. Those tabs connect to a circuit board in the motor that you can damage with too much heat.
Easiest way to do it for me is to tin the wires. Tin the tabs. Then solder the wires to the tabs. Be careful to avoid drips between the tabs. If there is a connection between the tabs you will have problems running the motor.
If you're using thick wire like 12 gauge you'll need a good hot iron. 80w or more I'd say. I've tried it witha a 40w iron before and while it sorta worked the solder didn't get very good penetration in the wire when tinning it. A more powerful iron makes it easier.