Making graffiti letters doesn't exactly come easily to everyone. It takes time and practice, but most importantly, it's key for the artist to develop their own style. The only way to do that is to start simple, and work your way up, discovering your style along the way.
Here's a simple tutorial to give you a place to start:
1. Write a word in big letters, maybe a nickname or your own name. Use a big piece of paper, and keep extras handy.
2. Take a thick marker and go over the letters multiple times until they are thick and fat and blend into each other.
3. Place a thin sheet of paper over your letters and trace the outline of the letters.
4. Ta-da, you have some basic bubble letters to get you started.
Obviously this is not a method that can always be used, and it's not the best type of method, but it gives you an idea of how to start. Pay attention to how the letters blend into each other and how they overlap. Also pay attention to local graffiti you find and figure out what style and what types of letters spark your interest.
Develop your basic skills first, then start slowly mimicking the styles you like. Try a variety of different words and letters, eventually you'll get better, just be persistent.
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