Wikipedia describes Mozart’s use of sketches and the keyboard in his compositional process:
Mozart often wrote down sketches, ranging in size from small snippets to extensive drafts, for his compositions. … Ulrich Konrad, an expert on the sketches describes a well-worked-out system of sketching that Mozart used, based on examination of the surviving documents. Typically the most “primitive” sketches are in casual handwriting, and give just snippets of music. More advanced sketches cover the most salient musical lines (the melody line, and often the bass), leaving other lines to be filled in later. The so-called “draft score” was one in an advanced enough state for Mozart to consider it complete… However, the draft score did not include all of the notes: it remained to flesh out the internal voices, filling out the harmony. These were added to create the completed score, which appeared in a highly legible hand.
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Mozart evidently needed a keyboard to work out his musical thoughts. … [He] had a prodigious ability to “compose on the spot”; that is, to improvise at the keyboard. This ability was apparent even in his childhood, as the Benedictine priest Placidus Scharl recalled: “Even in the sixth year of his age he would play the most difficult pieces for the pianoforte, of his own invention. He skimmed the octave which his short little fingers could not span, at fascinating speed and with wonderful accuracy. One had only to give him the first subject which came to mind for a fugue or an invention: he would develop it with strange variations and constantly changing passages as long as one wished; he would improvise fugally on a subject for hours, and this fantasia-playing was his greatest passion.”
Source: Wikipedia