Place the tile against the uneven wall and adjust it so that it is straight. If you have a small gap between the tile and the wall, you can trim it to fit flush.
Set up a scribe, or protractor, to a measurement of 1/4-inch gap. Run the scribe along the wall so that it marks the tile with a similar line as the wall.
Cut the tile along the marked line so that it fits along the wall. Use a router so you can follow the line even if it curves. Since the wall is uneven, you need to adhere each tile, before moving on to the next so that you know what part of the wall you are working on with each tile.