Transcribing in music refers to both listening to a song and writing down the score or adapting a song into sheet music for a particular instrument or ensemble. The process for either is the same, and often it is easier to make an adaptation after writing out the original score. Depending on the complexity of the song, this can be a fairly long process. However, with methodical work, it is not a particularly difficult task. Transcribing songs also provides insight into the songwriter's thought process in writing the song, as well.
Determine the basic song structure. Find out what order the sections (verse, chorus, intro, etc.) go in and how many musical bars are in each. Define these sections for the full song out on the sheet music of the transcription.
Transcribe the percussion track. Isolating out the lower EQ sections can be helpful. Having this section done first gives a skeleton to the transcription and is helpful for getting the note durations in other sections.
Transcribe another instrument, but be sure to choose one to focus on at a time. Adjusting the EQ and balance on the player can help to isolate that instrument. Since each instrument has its own pitch range and balance on the recording, these settings need to be played with.
Transcribe each instrument one at a time until the score is completed. Usually it is a good idea to go from simple to more complex, but this varies based on the song and transcriber.
Test the score out against the track to make sure there are no discrepancies. Alternatively, the sheet music can be programmed into a MIDI program and played to see if it sounds correct.
Adapt the major parts of the song to a desired instrument or ensemble if making an adaptation. Depending on the nature of the adaptation, the parts used in the transcription can vary. The main melody and basic bass line will almost always be used.
Most commercial sheet music or scores are not exact transcriptions and are usually simplified. However, these can be very good starting points and greatly speed up creating a more accurate transcription by providing the basic information about the song.
MP3 files are smaller in size than CD tracks because the audio is slightly lower quality. This difference might not be noticeable while listening to the song, but when transcribing it, there might be some issues as you isolate parts of the song. Often it is better to use a CD track than an MP3 or at least increase the quality of the MP3 file being used (programs that rip CD tracks usually have a quality option).
Each ear interprets sound slightly differently. Sometimes switching your headphones around or playing one side of the stereo in both ears can be helpful.