In general, to play the harp you want to hold the Wiimote level, with the buttons facing up, and then make purely horizontal movements. The most important aspect of harp playing is when you change the direction of your strumming, but in some cases the speed of your strumming matters as well. In general, the direction you start to strum in does not matter.
It is important to make sure to strum in one direction at a time; if you reach the end of your strum and have even the slightest movement back in the other direction, the game will interpret that as a new strum and penalize you if appropriate. It is recommended to make very controlled strum motions to avoid this pitfall.
Firstly, it is possible to start the trial gate songs early by beginning strumming at a particular point during the cutscene leading up to it.
When playing the harp to open a trial gate, the only important aspect of your playing is when you change the direction of your strum. It does NOT matter how slow or fast you strum, just that you change direction as the circle of light is changing direction. Ideally, you want to start a new strum not when the light is at its outermost or innermost point, but rather as it is reaching that point.
If you are slightly off in your timing, then you will not gain petals; if you are majorly off, you will lose one or more petals, depending on how far off you were. After the twelfth petal begins to appear, your strumming no longer matters, so you can freestyle a little if you so choose.
Kina's Duet is the final task you must complete for the chandelier repair sidequest at the Lumpy Pumpkin, and is required for 100% runs. Unlike Trial Gates, here the speed of your strumming does matter; you want to strum at a medium pace, so that you reach the end of your strum just as it's time to start the next one.
The music piece is in 4/4 time, and most strums are a half note (two beats) in duration. Roughly every four measures, the strums will shift to a quarter note (one beat)/half note rhythm. At the very end of the duet, there is one dotted half note (three beats) in the final rhythm. The rhythm never varies, so it's possible (and recommended) to memorize the song.
The middle section, when the audience is not waving its hands, does not affect your final outcome, so you can freestyle however you like during that section.