From the main page click on "New song". From this window, on the right, you will find the general parameters you have previously set. You can change the metronome speed (track speed) and the "percentage". This last parameter will allow you to define the "percentage of time the note can be played" It is easier to explain with an example: if the duration of the note is 1 second and the "percentage" is 50%, it means that you have 0.5 seconds to play the note. Obviously, you should try to play the note at the correct time (when the blue bar is on the note itself). This "percentage" parameter will give you some sort of "time tolerance" in your accuracy. If you set the "percentage" parameter to 5% (the lowest value), it means that your time accuracy cannot exceed 5% of the note’s duration. So, if the note duration is (for example) 1 second, the time tolerance with 5% of "percentage" is 0.05 seconds. So, you must be very precise!
I also recommend, especially the first time, to enable the parameters "show notes" and "tick metronome". Once you have chosen the metronome speed and percentage, you can start your first exercise!