The key is in the prep work, make sure all reference points are aligned (typically square). Normally the corners are built first, periodically checking the height with a transit, and the squareness with a framing square or the 3-4-5 method of squaring, and the walls are filled in after using a taut masons string. usually any pieces that need to be used are used in the corners so that you will have full block in the main part of the walls, which will make that portion go much quicker. make sure your footings are at a height that will give you the proper finish height using 8" tall block with the typical 4" cap block. Also, make sure the bond is correct (ie: the joints don't line up vertically). make sure you have the correct tools and like Jay J said, the correctly proportioned mortar mix.