I mean all aspects: atonal dictation, two-part dictation, harmonic, etc. I'm practicing sight singing by myself.
Becoming good at taking dictation is a huge undertaking. It sounds like you're doing the right things with sight singing, which is really the key to being able to transcribe music well.
In college, beginning dictation tests went like this: listen to a short excerpt 4 times, have a pause, then once more.
For the piano transcriptions of chorales they usually used, the professors recommended a strategy like this:
- 1st time - try to catch the harmony, part of the bass line, and melody
- 2nd time - fill in the rest of the bass line
- 3rd time - fill in the rest of the melody
- 4th time - try to catch the inner voices
During the pause, check the harmonic analysis and get the inner voices sketched.
- 5th time - make sure that everything is correct
Remember that the point of dictation is not 100% accuracy, especially at the beginning. The most important thing to figure out are the chords and outermost voices.
For practicing on your own, what I've done (especially to pass that class!) is:
- copious amounts of sight singing
- solfeging (in my head, not aloud) ambient music
- analyzing harmonies of ambient music
I'm a drum guy, so rhythmic dictation comes pretty naturally, but don't forget to work on intuitively hearing complex rhythms. Understanding the rhythmic structure often makes the harmonic and melodic parts come together for me.
If you're doing these things frequently, you'll be practicing all the aspects of taking dictation without actually doing any writing.
For checking yourself, I'd suggest listening to pieces in the public domain and transcribe short excerpts (like 4 bars to start with). Find the music on IMSLP or elsewhere.
You will probably be best at transcribing what you are best at playing. If you're just starting you may find you're most comfortable with music on the instruments you play. For me, transcribing a piece played on marimba is easier than a similar piece played on piano, I think because the mechanics and body motion make it easier to visualize what's going on. You may find it helpful to work through transcribing a piece in short chunks and then learn it.
Dictation really isn't magical, unless you're Mozart or something; it's a learned skill. As such, practice in short sessions, because it will be very taxing at the beginning.