How To Pick Up A Melody By Ear
Your dedication to developing the acuteness and sensitivity of your ear can make a world of difference in your progress as a musician. So where do we start?...
There's an old saying, "If you can hum it, you can play it". This only means if you know a melody really well (it's etched into your brain), then you can find those pitches without too much problem on the guitar fretboard. But..., the catch is you've got to really know the melody, or else it's likely trying to repeat a message that someone's mumbled to you. So pick a tune you know inside out. There's nothing wrong with nursery rhymes or folk songs (why do you think they've been around for so long?).
Then, the next thing is to unpack your Unitar. Your Unitar is a one-stringed guitar. This way when you're figuring out the melody by ear, you won't have to deal with the distracting tonal differences you hear from string to string.
Next hum the starting note of the melody and peck around on the guitar to find the note that matches your hummed note (The "Hum-Match" Method). Hum "doo" or some other monosyllable. When your ear detects the matching note, put that same pitch on a string where the note is fairly centrally located. Remember, we're only using one string, so we need to be able to move up and down a bit on it. From there you can continue to "Hum-Match" or just let your internal ear take over. Try to listen to what you play objectively, as if you're listening to someone else play. When you get the melody right it will really sound like the song and jump out at you---the familiarity will be startling. If you're off on a few notes, you'll know---don't accept this and give up, but keep pecking. The more you practice figuring out melodies, the easier, faster and more direct it will become. When you've got the complete melody on your "Unitar", you can then transfer the notes to more than one string.
Sometimes a melody might be harmonized (more than one note sounding together). Still, depending on the key, one note will clearly predominate to your ear, and that is the note that you want for your single-note melody line.
The important thing to remember when you hear a melody is that the notes are never hidden from you---they're right there in front of you. But until you've mentally internalized those pitches, it'll be next to impossible to pick them up by ear.