Being able to play by ear requires EAR TRAINING. After going through this, you can transcribe songs easier, WRITE easier, and among other things. It sounds impossible to achieve, but remember this; Music is a langugue; it can be read, written, made, and expressed. You listen to some one say a sentence, and you can repeat what they said, right? Well, doing that for the guitar is the same idea. You didnt learn the english langugue in 6 hours... It takes time, plus the things I previously mentioned, and many more...