think of letter structure instead of bubbles, graffiti is an art and a style so you need to think a bit more on composition. equality of space how it flows and if it looks cool to your eyes. thicker more even lines always help. and remember when you are just starting out, and even beyond that, less is more. DON'T BITE PEOPLE, but gain influence from writers they know what they are doing. now this doesn't mean take things from them, but look how they evenly use space in their letters how they sometimes use repetition to create a flow. lastly work on your speed, don't rush, but don't waste your time, smooth consistant lines.