Here you can find some useful tips for best gameplay.
Tips for Gameplay
How to play the Piano Game
You just go to the place with the lowest note. Walk to the yellow 'bars' floating above and find the one closest to the bottom of the screen (closest to the piano) and hop on it.
The next lowest yellow bar will light up and you have to land on that key without touching another key. As you hit it, that yellow bar vanishes and the next one lights up. Continue through the song.
The text above explained it like it is harder than it really is. Just start hopping on the key with the lowest yellow bar and continue bouncing on the right keys.
Progress bar and secret room
The bar represents your Violence (red) and Wisdom (purple). Violence is filled by taking damage and spending health. Wisdom is filled by not taking damage and spending crystals. Crystals chests, doors, and the big crystals can be shot (by you or the enemy) to turn them red where they cost health to use, filling your Violence meter.
Once full, you can teleport to the Devil Room (red) or Angel Room (purple). In the Devil Room, you spend one heart container to get a choice of three items. Angel Rooms give a choice of two items but don't require to sacrifice a heart container. Can only pick one in either room. In my experience, Devil Rooms tend to offer more offensive based items while Angel Rooms offer more utility based items. I personally find the Angel Room better because they've given me flying a couple of times and I value that over damage. You may feel differently!
To make sure it doesn't seem like I'm trying to say that Angel/Wisdom rooms are the best. I have also gotten garbage/situational from them such as Magic Wand (chance to drop crystal when you get hit). The Devil/Violence room gives more options and that on its own is worth something. Similar to Isaac, I would suggest Devil Rooms are more consistent even at the cost.
About purple crystals
Spending a crystal increases your Wisdom (purple). Hitting the crystal to turn it red than touching it to spend HP increases your Violence (red). This is the bar in the bottom right. Once full, you'll get an Angel Room for purple or a Devil Room for red. So far, Angel has been a choice of one item out of two. Devil room is you sacrifice a heart container to get one item out of three.
They seem to have special items in these rooms but also will have items you can find in any other item room from my experience so far. Edit: This all also works on crystal doors/chests. They give some Wisdom is you spend crystals to open them, or Violence if you shoot them then give HP.
Unable to purchase upgrades at bar
You have to load the game without a controller. Use your mouse and keyboard to allocate points.
How to get past lasers (block entry to a room)
Walk through them. You will lose half a heart each time you pass through one, unless you have environmental hazards immunity.
How to use Ritual Mask
You can use it by any method (using or active, opening shop doors, opening crystal chests... etc). Once you have consumed a total of 5, you'll regain a heart.