Turn down the volume of the keyboard amplifier. There is no need to unplug or turn off any of your equipment.
Plug the first cable into the "Audio Out" jack of your synthesizer. Plug the other end into the "In" jack of the reverb pedal. Plug one end of the second cable into the "Out" jack of the reverb pedal and the other end into the "In" jack of the keyboard amplifier.
Turn the volume on the synthesizer to a moderate level, about 50 percent. Play the synthesizer while you gradually turn up the volume on the amplifier until it reaches an acceptable level.
Step on the reverb pedal switch to activate it.
Adjust the "Dry/Wet" control. The more "wet" the setting, the farther away the synthesizer will sound.
Adjust the "Length" control to set the reverb tail. The reverb tail is how long the effect takes to die out.
Adjust the "Room" or "Effect" control to set the basic character of the reverb. Possible settings include spring reverb, hall reverb and plate reverb. A spring reverb sounds like a guitar amplifier reverb. A hall reverb sounds like a concert hall. A plate reverb sounds like a particular electromechanical reverb effect used in recording studios. Experiment with these various settings until you understand their character.
Experiment with various synthesizer sounds running through the reverb pedal. Try different combinations of synthesizer sounds, reverb effects and playing styles. For example, string sounds through a hall reverb with moderate settings can create a lush or anthemic sound -- and if you turn the "Dry/Wet" mix all the way up, you will have an ambient music effect. Extreme reverb settings will make an atmospheric sound. More moderate reverb effects can help to smooth out a staccato synthesizer sound. Experiment, and find your favorites. This is the best way you develop your own sonic style.