A 12-volt rheostat is a coil of resistance wire with a contact that can slide across the coil. Depending on where the contact is along the coil, the resistance between the contacts will be different. This forms a variable resistor. These types of resistors usually have two or three terminals. For the units with three terminals, usually only two of the three are used at a time. If you label those three terminals 1,2 and 3 looking left to right with the pins pointed down, you would use either pins 1 and 2 or pins 2 and 3.
Place the battery on the work table. Strip 1/2 an inch of insulation off the ends of all three wires. Connect one end of the first black wire to the negative terminal of the battery. Connect the other end of that wire to pin #1 of the rheostat.
Connect one end of the second black wire to pin #2 of the rheostat. Connect the other end of this wire to the negative terminal of the DC motor. Connect one end of the red wire to the positive terminal of the battery and the other end to the positive terminal of the motor.
Use the rheostat to control the motor speed. By turning the rheostat, the resistance of the rheostat changes, and that in turn changes the voltage at the motor terminals. As the voltage at the motor terminals decreases, the motor slows down.