What I do is write down the chord progression & structure. I find melodies easier to "just remember" and play than to jot down in staff. Does not help in the long run, I do forget stuff.
Lead sheets... You can buy books with transcribed pop songs for guitar or piano. Browsed through some, always found errors or blatant simplifications that give wrong results. Some say this is done to avoid copyright, but I don't buy it (sic)
How to check you got it right? I used to input the chords into BandInaBox and let it play it. Then to find out the closest style insists on playing all chords with the seventh which it should not do in that case. Oh well...
I use my ears, nothing else. Its a gift. If stuff is masked, its not important for transcribing.
What's more important is the ability to write down what you hear in your head, without having a recorded example. I imagine things my fingers cannot play. Should practice more often.
Here's a story I recently found Bruce Swedien told about Quincy Jones
SoundOnSound.com wrote: Quincy and I first worked together with Michael Jackson on the movie The Wiz. We were living together at a hotel in Manhattan, and we would go to Studio A at A&R Studios. We had a big session at noon on Monday to record some of the music with a big 70‑ or 80‑piece orchestra, and we had to leave for the studio at 10am. The night before, Quincy and I had guests at our hotel for dinner, and Quincy still hadn't even started on the orchestration for the opening titles. I was getting a little nervous, but he said not to worry about it. At about four that morning, I woke up and noticed under my door that all the lights in the apartment were blazing. There's Quincy at the dining‑room table with a billion sheets of manuscript paper, and he was writing orchestrations. I said 'Quincy, we've got to leave soon!', but he just said 'Don't worry about it' so I went back to bed.
At about nine o'clock I got up again, and Quincy said to me 'I'm all set'. There wasn't even a piano or a guitar in the apartment; just Quincy and his manuscript paper! Off we go to the studio, and Quincy hands over his score to the copyists. He didn't even want to conduct — he'd hired a conductor because he wanted to be in the control room with me. The conductor gave the down beat, the orchestra played the entire overture, and there was not a single note out of place. It still gives me the chills to think about it!