Please note everything I post here, are only my opinions and research... I am just trying to help, not earning a single cent here...(
You can choose not to read any of these if you don't agree with me..
I am sorry if I don't really remember any musical term...(I have quitted playing piano since 4 years ago...don't think my memory can last tt long,
Steps in playing by ear
1) Listen to the song for as many times until you can hum the main tune/melody
- Recommended to use headphone or earpiece
- Preferably with a piano or piano keyboard beside you (you need to find the main chord which it is played in)
2) Play the main tune using RH only
- Play in single note 1st, don't rush things by trying out the chord.
- Make sure you are confident that you rmb the tune and how it is played before proceeding on to the next step..
- Preferably to play it a few times to get yourself familiarise with it
3) Analyse the song
- What type of genre of music does it comes under? (R&B? Hip Hop? Rock? Pop?)
- Catergorise it to some music which you have played before ( For example, if you played 2PM's 'Again and Again' before, you will feel that 'Heartbeat' is also similar)
- Listen carefully to spot some of the main instruments' tunes in it (The main ones are usually the piano, drum, guitar and bass)
- Understand how these instuments were used in the song.(some of them are really important at times when the vocals are not singing)
4) Play RH in chords along with the music
- Play along with the music (helps you to check if you are moving at the correct way, and is your RH chord similar to the chord used in the song)
- Try it a few times (the chords are usually similar , especially when it come to the repetitive trend in kpop music)
5) Try out your LH now
- Experimental stage (Depends on the RH chords which you have just found)
- Try with simple playing (single notes again)
- Requires technique and good memory of the scales
6) Put them together
- If you reached this stage, you are basically almost at the finishing point..
- Requires alot of practices to familiarise
7) Decoration time
- Decorate your song with some techniques and also other special tunes you have listened to in the song
8) Finish!
- Now is the end of the whole fun but tiring process :D
- You just require some more practice :)
*Note :
You don't really need to follow this process everytime.It only serves as a guideline. Just find a type of way that is comfortable and manageable for yourself!
As you become more and more better in playing by ear, you can just proceed to play by chords directly instead of having to play single notes!!

