To use this method you will need something to play back a recording on (CD/mp3 player), some manuscript paper and a pencil.
Firstly pick a piece of music you like or know well. Using your music player, listen very, very carefully to the music – try to pick up the pattern of the notes and rhythms. Listen to this section of music several more times until you can sing it note for note from memory.
Once you can sing the section from memory go to your instrument and try to play this music. Trial and error is OK so long as you are accurate.
Once you can play ther section comfortably on your instrument write it down on some manuscript. Pay attention to all the details, dynamics, phrasing etc Repeat the process for a different section of music.
You may find this tricky at first but make sure you persevere. The more you do this the easier it will get!