Controlling fan speed with a rheostat
You put the resistor in series with the Fan. Keep in mind to not exceed the maximum current (around 300mA) of the rheostat.

There are no positive or negative terminals on a resistor. Internally, it's just a resistive plate with a kind of "wiper" connected to the middle terminal.
So you connect one of the end terminals and the middle one.
For example
+12V to left terminal and the fan to the middle terminal. Than you can change the fan speed.
If you think you have to turn in the wrong direction to increase/decrease the speed, just connect the +12V to the other terminal.