Open it in your VST host application, connect the host midi input to the MidiTransposer’s midi input and its midi output to the midi input of a Vst instrument. Press and hold a button of your midi keyboard sending a CC message #110, set the desired transpose interval with reference to the C3 key (midi note number 60) pressing the corresponding (upper or lower) key, then release the button: the chosen transpose interval is shown in the MidiTransposer’s GUI in terms of upper or lower (-) semitones.