How does one go about learning/improving ear playing after having learned to play exclusively from notation?
It's about training your brain/ear/fingers (or voice) to make connections, and a multi-pronged approach will be best.

First of all, spend time with your instrument and very simple tunes (nursery rhymes were suggested) and try to pick them out bit by bit.
You learn to recognize intervals and reproduce them only by doing it, and of course doing it badly at first. You then move on to recognizing chords and chord changes the same way. A little theory will help narrow down the possibilities when it comes to chord changes, too.

Second, if you're one who learned the "dots" in such a way as to make you an organic playback device, you will be well served to learn the theory to go with it. Seriously, theory applied to the music you already know will go a long way to helping you understand -- and later, hear -- the aural implications of the various bits that go together to make a tune or chord progression.

My problem has always been the reverse. I got very good at ear playing very early, and scamped the discipline of learning to sight play. It's hard work to remedy that, and may be just as hard to learn to use your ear after years of being a servant of the dots.

-Glenn