Sit on the front half of the bench, back straight but relaxed. Elbows level with the keys or slightly above — adjust the seat height if needed. Feet flat on the floor.
Your distance is right when your outstretched fists reach the fallboard: the elbows get room to move.
Hand shape
Imagine holding an apple: fingers rounded, wrist loose and level with the forearm. Press the key with the fingertip pad, not a flat finger.
Fingers are numbered from thumb to pinky: 1 — thumb, 2 — index, 3 — middle, 4 — ring, 5 — pinky. These numbers — the fingering — are printed under the notes.