Many grand pianos feature a lock which can be used to hold the lid down over the keyboard, to prevent people from playing it. This can be a convenient way to prevent unwanted performances, if you own the piano and have the key. If you've lost your key, however, or are attempting to play a guerrilla sonata on a friend's piano, being able to pick the lock is a very handy skill to have.
Insert the torsion wrench into the keyhole and twist it. This will apply torsion to the lock. Maintain this pressure during the entire picking process.
Stick the hook pick in beside the wrench and feel around for the tumbler. Press upward on the tumbler closest to you until it reaches the correct height. If there are multiple tumblers, the lock will turn slightly and allow you to engage the next tumbler. If the piano has a single-tumbler lock, the lock will open.
Carefully remove the wrench and pick. Lift the piano's lid to play it.
Most Steinway piano locks are virtually identical, which means that you can simply buy a replacement key or use a key from another piano to open yours, instead of purchasing a set of lockpicks. Other manufacturers may have a similar situation.
Consult the laws in your area regarding the possession and use of lockpicks, especially if you're attempting to open someone else's piano. Piano-lock picking is a special case, since the only theft picking the lock could possibly facilitate is that of silence, but it's still far better to be safe than sorry.