You can write a piano idea as music notation before you have a complete arrangement. Start with the phrase you want to hear, choose a readable rhythm and use playback to check it. The first useful score might be only two bars long.
Open a blank piano track to follow along. The piano music notation editor gives you sheet view and a piano roll for the same part.
Start with a melody you can sing
Play or sing the phrase once, then find its first note. Enter the melody before adding accompaniment. This gives you a line to recognise in playback and a simple way to catch a wrong pitch.
For a first example, write C4, D4, E4 and G4 as four quarter notes in a 4/4 bar. C4 is middle C. Listen through the bar and count four beats. The example is deliberately small: you can check every event without searching across a long score.
Choose the key signature and time signature for the musical idea, rather than changing them to make an accidental or a bar line disappear. An occasional note outside the key can be part of the phrase.
Give the rhythm a clear shape
Music notation describes duration as well as pitch. Decide which notes are short, which are held and where the phrase needs silence. A quarter note followed by a quarter rest occupies the same two beats as a half note, but the result sounds different.
Use a simple count while listening. In 4/4, a whole note spans four quarter-note beats, a half note spans two, and an eighth note spans half a beat. Start with those relationships before adding a more involved subdivision.
Read across the bar line. A phrase does not have to stop where the measure ends, and the last note of one bar may lead directly into the next. Check the connection in playback instead of judging the two bars separately.
Add the accompaniment beneath the melody
Begin with a held bass note or a simple chord. Play the melody with it and listen for the harmonic change. Add the remaining accompaniment notes only when that foundation works.
For the C–D–E–G example, try a C3 bass note beneath the melody. Then compare a held bass note with repeated bass attacks. Both use the same pitch, but the second version creates a different rhythmic texture.
Keep track of the voice you are writing. A moving top note can sit above a sustained chord. The piano roll is useful for checking those different lengths when the sheet music alone leaves you unsure about an overlap.
Check the grand staff as a reader
Treble and bass staves work together in piano notation. They help distribute the range of the instrument across a readable score. The lower staff is not an absolute rule that every note must be played with the left hand; use the musical phrase and your fingering when deciding how to play it.
Read the score away from the keyboard for a moment. Can you follow the melody? Are repeated rhythms written consistently? Does a rest explain a gap more clearly than another short note would?
Save a version you can continue
Use playback after each meaningful change, then keep the version that expresses your idea clearly. Save the project to your library or export a native project file. MIDI is useful for production; MusicXML is useful for continuing notation work in another application.
Start writing piano notation. If the idea already exists as a recording, use piano transcription rather than entering every note from memory.