Harmonising is playing the right notes of a melody to fit the right chords of an accompanying rhythm to the same timing. In playing a piano or other keyboard instrument, the right hand harmonises with the left. The notes played with right hand and left hand at any moment match the notes of the same chord. Yet the notes played at any moment are often different from each other and so are the intervals between them. Still, notes or chords played with one hand are kept in time with those played by the other. Timing is measured in beats to the bar. It may be 2/4, 3/4, 4/4, 5/4 or some other timing. The upper number of a fraction is the number of beats played in one measure. The lower number of a fraction is the duration and number of notes which make a full measure. So 3/4 time is three notes to the bar with each note being a 1/4 quarter note. A quarter not is the pulse for that timing. A quarter note lasts for an interval of 1/4 of a full note. One full note takes up a whole bar or measure in 4/4 time. Playing in time is keeping to the pulse of each measure.
In 4/4 time, or common metre, for example, the pulse is counted 1, 2, 3, 4. That is four equal beats to the bar. Learn to play chord C or just note c at four quarter intervals to the bar. Let each interval be one second long. So each measure here is four seconds long. Say one, two, three, four to yourself as you play. To help slow the tempo, say 'and' or 'an' between each number. That is one and two and three and four and one and two and three and four and ...
Playing in four/four time can be notated as four quarter notes in one measure, like this.
De1
Turn that regular pulse into a rhythm by changing one or two quarter notes into notes of longer or shorter duration. Let's turn each second quarter note into an 1/8th note with an 1/8 pause or silence. The notation for that is:
De2 
Two quarter notes, two eighth notes plus two eighth silences adds up to four quarter notes. So the timing is right; four beats to the bar. Practise playing chord C with the left hand to that rhythm.
Learn to keeps a steady, even pace or tempo. Keep time while you walk like this: One left foot, and right foot, two left foot, and right foot, three left foot, and right foot, four left, and right. Count 1&2&3&4&1&2& so on.
This time turn the pulse of 4/4 time into a rhythm by playing the third beat as two 1/8th notes or chords, like this.
De3 
Practise joining the first two beats together as a half beat.
De4 
Play different rhythms as triad chords with the left hand.
Practise playing chords to this rhythm, with a half note joined to a dot single note for a count of three and an 1/8 note and 1/8 pause for the fourth beat.
De5 
If the piano or organ cannot sustain the first note for a count of three, just let the rest of the note count as a silence before playing the 1/8 note.
Another easy beat is three quarter beats followed by a quarter pause in four/four time.
De6 
Keep steady time for each rhythm. Do not let that rhythm lapse from three quarter notes to three 1/8th notes. Do not let a half note lapse into a quarter note.
De8 
So vary the basic four beats to the bar by: playing a silence instead of one beat; playing a half beat instead of two beats; playing two eighth notes instead of a beat; or playing any other rhythm.
Practise playing one rhythm after another and repeating them together as a combined rhythm, like this for example.
De9 
Try playing different combinations of different rhythms.
Practise playing the same rhythm with different chords in a progression in the key of C or another key. A progression of chords begins and ends with the root chord, such as chord C in the key of C.
While playing rhythm chords with the left hand, play melody notes with the right. Improvise with different notes with the right hand. Notes played with the right hand match those which make up the chord being played but are played an octave or more apart.
Keep the fingers of the right hand in the first pattern, that is thumb, pointer, fourth and little finger or fingers 1,2,4,5. The middle finger 3 is kept mainly for some sharp and flat notes.
Practise playing the same chord with both hands at the same time. The left hand uses patterns one, two and three for chords and the right hand uses only pattern one. Playing chords with three patterns keeps the left hand deep bellow note middle C on the piano. It leaves the right hand free to play melody notes higher than middle C over two or more higher octaves.
Remember the holistic patterns for playing chords most efficiently over all. For example, do not jump the left hand from chord Dm up to Em as that takes the hand out of the pattern and away from the other chords. Instead, pivot to the left from chord D to the deeper chord Em. From Dm played with the third finger position, pivot on the pointer finger to form chord Dm6, then move the pointer to form an Em triad. Other chord changes are made with one movement, not two.
