Do not rapidly tap with no sense of rhythm. When you tap to the beat, you will earn a note multiplier based on how well you are performing. If you do not miss any beats, your combo multiplier will continue to grow. Once it is in the x300 range, your earnings will be significant.
Multiplier
You may get a beat multiplier as a random reward. It automatically increases your tap beat multiplier for twenty seconds. However by watching an advertisement video just before you get it, you can double the multiplier.
UFOs
A red warning and sound will indicate the presence of a UFO. Spin your planet in the direction the warning indicates to find the UFO, then tap it to destroy it before it disappears. This must be done to prevent the UFOs from decreasing your building efficiency. You may also earn gems by doing this.
Upgrades
Consider investing a lot of building efficiency into the recorder if you will not always be playing actively. The recorder generates notes for you, however it has a limit that depends on how well it has been upgraded. Collect your earnings regularly to avoid losing them. Displayed under your total notes is a red number that indicates your "building efficiency". This corresponds to how many notes your buildings can generate per second. A higher number is better. To increase it, upgrade your buildings. Each building you have increases the building efficiency. You can view how much each type of building earns by looking next to it in the "Building" menu. Upgrade everything equally to keep your building efficiency high.