How do I design a half sleeve tattoo?

You should have your artist involved from the very beginning once you have an idea of what you would like.

Notice I said "your artist."

If you don't already know who he/she is, you're approaching this the wrong way. I had four or five tattoos (including rework) from a random selection of "artists" before I finally got an actual piece of art from the only artist I will now ever let do work on me. I will fly halfway across the country (FL to TX) to have work done by the guy I use.

You should spend a lot of time researching artists in your area, view their portfolios, meet them and chat a bit (do NOT try to bug them while they are working). The best source of info for where to find an artist is bartenders in your area. Ask around, most shops have a reputation, and usually it's for good reason. Find someone you're comfortable with, pay them a retainer (usually $20-$50 depending on the size) and work with them to create something amazing. They're usually as excited as you'll be about creating some new art. It can take a few weeks, but they'll show you an initial drawing, you can add input, and eventually you'll have a fantastic piece of artwork to share with the world.

Good luck!

i would be hesitant to get any half sleeve design found online. generally people who are going to get sleeves go to an artist and have them create a custom piece of art. that would be the only place i would consider going to find a tattoo design.

Take in as much reference as you can to a good artist. They will then draw up the design for you :)