A half sleeve tattoo is a tattoo that wraps around either the upper or lower half of the arm. Since the arm is the most popular place to get a tattoo, you do see a lot of these around. How do people design their tattoo?
First they decide where they want the tattoo placed. Do you want it on the upper where you'll be able to hide part of it with a short sleeved shirt or on the lower arm where it'll be relatively visible at almost all times?
Next, for half or sleeve designs in general, it's good to work around a theme. By having a single theme, your tattoo will flow and fit together in ways that a tattoo without a theme would not.
The following step would be to find small pieces to include in your tattoo. By including small designs into your tattoo, you'll further build flow and create a custom tattoo.
Finally, consult a professional tattoo artist. You may want to do this in the beginning to find an artist who creates designs that fit with your wanted tattoo and who will work with you to design a completely custom half sleeve tattoo design.
A tattoo can be a great piece of artwork or it can be a piece that misrepresents the wearer. It's important to take the time to design and choose the right tattoo for yourself. You'd hate to end up with a piece you despise on your body. Please make sure to consult and to have your tattoo placed on your body by a professionally trained tattoo artist.