Write a piece of music. Make it the greatest, most beautiful, transcendent piece of music you have ever written.
Without playing or singing this piece of music to anyone else, record it onto some physical format (be that a paper score, magnetic tape, or whatever) such that your recording contains sufficient information to fully reproduce this piece of music to your satisfaction. Make no copies of this recording. Remove any traces left by the process of production. Alone, without witnesses, and without wrapping it in any kind of casing or covering for protection, bury beneath the ground the physical object on which this recording is held.
Never reveal to anyone else the nature of this piece of music that you have buried. Never perform it, never explain a word about it. But on regular occasions (say once a week, once a month) visit the site where you have buried it and leave flowers or pebbles or some other token. As you are leaving your offering - and only then - allow yourself to think about your piece of music. Run through it in your mind soundlessly.
Leave instructions in your will such that after you have died and a sufficient amount of time has elapsed, your next of kin is to exhume your piece of music. They must not clean it or make any attempt to otherwise restore it; they have simply to play whatever it is they find.