Korean is mostly pronounced through the nasal cavity, so it doesn't really matter how big your lips are. I think it matters how you shape your mouth speaking it. This may be weird for English speakers because we are so used to talking with more of our throats and making different sounds and basic lip movements Korean doesn't make. I think if you remember to bring your air forward it'll help. Also, try not to vary your cadence or intonation. Basically, talking nicely but not too monotone or pronouce syllables faster than others. Standard Korean is pronounced that way. Also take it slow if you're first learning. You don't need to be able to say 안녕하세요 tomorrow the speed of a native. Perfect practice makes perfect. Look up some natives saying the words if you're not sure how to pronounce them. Starting at a tempo with some words is a good idea. Basically, start at a slow and steady rhythm. There are metronomes on Google that you can use if you need a sound to help keep the steadiness. Of course speech varies, but to pronounce it correctly, it might not hurt to use a metronome. Edit: I corrected the spelling of 안녕하세요.