Retaining walls are often a homeowner project, especially if just for landscaping, and a clear understanding of how the retaining wall works, is key to building it to last. I come accross them on almost half of my home inspections in St Louis, Mo.
It's obvious that these walls are most often on hillsides, but, what is less obvious is the way they control and withstand drainage coming down that hill, as well as freeze / thaw cycles.
Starting at the bottom, it is always easy to find where the trench was dug out too deep, and filled with loose dirt. Loose fill compacts, and there will be dips in the wall.
Under the wall, for leveling the block, I like to use a thin layer of dry 1/2 & 1/2 sand / portland fill over a nice level hole. While this layer should not be to thick, or used as substitute for solid undisturbed earth, it makes setting/leveling the individual stones in the first course easier. Manufacturers vary on suggested depth below grade for first course, and you should always follow their recommendations, as well. Stepping the wall base coouturse up while keeping the stones level can be tricky, but worthwhile. String lines will help. A transit will help even more.
The main thing that has me writing this article, though, is the back side of the wall. The wall must be able to drain, meaning that drain cloth to hold dirt away, possibly a drain tile (perforated pipe) in a sock in the bottom, with an outlet through the wall, and back-filling with gravel and no dirt directly behind the wall from the very bottom almost to the top, is recommended in most cases. If the wall holds moisture, it's a dam, holding even more weight. Additionally, if it freezes, it will be being pushed a small amount every freeze / thaw cycle, over and over again. If a (landscape stone) wall is standing straight up and down, it's probably being pushed, as most will step back slightly into the hill.
Worth mentioning twice, make sure you read and follow the manufacturers recommendations, warranties are almost always dependent upon them being explicitly adhered to, and it will look better and last longer.