In this case, we're building a brick wall that's perpendicular to an existing wall, so we have to make sure that it fits properly. After you've laid a solid foundation for your bricks, work out how many courses of bricks you want. Mark out the height of each course on a piece of wood, so that you can use it as a guide to make sure it's straight and so you can use this as your gauge rod. Put a straight piece of wood next to the wall with a spacer behind it, so you can feed the string line behind it and clamp it to the wall. Use a spirit level to make sure it's straight.