A variable resistor called a rheostat has two connections, and hooks up in series with a voltage source to vary its level from the full voltage to nothing. Rheostats appear in thousands or electrical and electronic applications for a multitude of uses. When you vary room lighting with an adjustable knob, you use a rheostat. Since there is no specific use, you must have electrical skills to use a rheostat not already installed in a device. You must know how to connect wiring safely, and know how to select and use all applicable tools and hardware.
Shut off the voltage source to the device that you wish to control with a rheostat. Since rheostats come in a huge variety of sizes, this could be a tiny electronic circuit or a huge motor.
Connect one wire of your rheostat to either one of the two leads from your voltage source.
Connect one lead from the electrical device you want to control to the other lead of your rheostat.
Connect the other lead of the device you want to control to the other lead of your voltage source.
Turn the power to you voltage source back on.
Vary or turn the rheostat until your device reaches the level at which you want it to operate, be it light intensity, audio volume, heat, the speed of a motor or any other control.