Tiling a job

Hey guys. I have to tile a job and I am worried about it coming out clean and straight. I have to piece together two pieces of 15" material on a wall. Matching up the edges won't be bad, I am worried about applying the material as straight as possible. Do you have any tips or tricks that would be helpful to make sure I don't screw this up. This could be a job that gets my foot in the door with a company that is interested in more signs. Thanks.

This is something I send out with transfers for people to install themselves.
The only trick is to make sure the second piece overlaps and is nice and even with the first top and bottom. For that I use a 4' level across the top.