That BTS7960 module is not a step down converter, however you can use it to run that 18 V motor at less than 24 V.
The BTS7960 needs a PWM signal (which you can generate with the Arduino), if that signal has a DutyCycle of less than 18V/24V = 75% then the motor should not notice the difference compared to feeding it 18 V DC.
If you go above 75% then in essence you're feeding it more than 18 V and that could damage the motor if sustained for too long.
Note that the currents which are going to flow between the supply, BTS7960 module and motor are significant. I mean, you MUST use thick cables which can handle that current and do not drop too much voltage. See the table on this page to see what AWG you need. If possible or when in doubt, use cable with more/thicker copper. Also make the cables short. It is better to have a long mains wire (to the supply) instead of long 24V cables.