Ah, it's supposed to be on the beat, when you see the corners of the screen flash red. If you pause with P, you can make the flash a little more solid if it's hard to see. But also I suspect the timing between the audio and when the game thinks the beat is is probably off, so you might have to go into settings a change the offset a few times to get it right. Given how different it seems to be depending on if I play it in the Godot editor or if I play it on the web build via itch, I have no idea how it'll behave on anyone elses PC. Oh, but also if you just want to make it easier then change "beat input window" coz that's how far off beat it will accept inputs, too. Maybe 0.14 is too harsh, given I found it fair but I've played this game a lot while coding it, plus I used to be a drummer.
OK, I don't really know how to do a tutorial for it, but I've hastily scribbled a visual guide to try to explain when you're meant to be pressing the buttons, maybe that'll help? As for easy mode, make the beat window higher and you'll have a wider range of time to press without it saying you're early or late.