I think that one of the most common methods which is widely promoted on web sites, is to look and learn the characters visualy before listening to Morse Code. On a lot of Morse Code learning sites there is a chart with dots and dashes that you can memorise and this is the worst possible thing that you can do. You are learning the Morse Code LANGUAGE visually which does not work.and will only cause you problems in the future. LISTEN TO MORSE CODE, don’t read dots and dashes.
Last year at the National Hamfest, I was testing a candidate at 12wpm and they struggled as they were trying to visually imagine the character. I told them if they were trying to visualise the character in their head then it would be a waste of time taking the test. Using your ears works, using your eyes only adds another brain process and you will be lucky if you get beyone 10wpm.