How to use the online timer
Half an hour at the piano is enough for scales, a section of whatever is being learned, and a few minutes of playing something for its own sake. The instrument has one advantage over most others in a flat: a digital piano with headphones removes the neighbours from the equation entirely, which is why so much practice here happens late in the evening.
Hands separately before hands together, always, and slowly enough that no note is a guess. Most of the half hour should go on the four or eight bars that are actually causing the problem, not on the pages either side of them. Keep this tab open while the count runs, on a phone on the music desk or a laptop on the lid — closing it ends the countdown.
Fingering written in and then kept is worth more than any amount of repetition, because inconsistent fingering means relearning the passage every time. Sight-reading something new for five minutes at the end of each session, badly and without stopping to correct, builds a skill that scales and pieces never touch, and it costs almost nothing to include.
Common questions
Hands separately or together?
Separately first, always, and slowly enough that no note is a guess. Putting hands together too early usually means learning the mistakes in both at once, which takes longer to undo than to avoid.
Should I write the fingering in?
Yes, and then keep to it. Inconsistent fingering means relearning the passage every session, which is why a written-in and repeated fingering is worth more than any amount of extra repetition.