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When building a retaining wall with stones, your best bet is to stick with flat ones. It is possible to do with rounded rocks but much trickier to get them to stack properly. With flat rocks you can obviously layer them without much difficulty. But, this doesn't means it's easy.
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Where necessary, slide small flat rocks under your main rocks to keep them from wiggling. Odds are your flat stones are not completely flat, at least not all of them, and sometimes they need a little boost. And then sometimes they'll need to be trimmed down, a hammer or large solid rock will come in handy here. But always try to exhaust all your options before adding or taking away, this is why we got so many rocks to begin with. It's a trial and error process and eventually one of your stones will fit perfectly, you just have to turn and flip them a lot. Don't worry, rocks don't get dizzy. At least I don't think they do.
Two or three layers should complete your wall. Anything taller might require two rows for each layer and some much larger rocks. So if you need to go higher, keep in mind that you'll need larger stones for your lower layers and making the wall sturdier becomes a necessity. Make sure none of your rocks are wiggling at all, on shorter walls a little wiggling isn't the end of the world. It's not like you're going to be walking on it. Unless you are, in which case you will need some super flat and thick rocks. But the average retaining wall is great for sitting on if you've constructed it correctly.
Here's where our paths may diverge, depending on what your wall is for. Mine is for more rocks to go inside, the wall is simply to keep them from spilling over into the yard. So I start with flat rocks for the base layer inside, see, those extra rocks are really coming in handy now. Cover as much of the ground with flat rocks as possible, the more layers like this the better, it'll stop the weeds from bursting through. Then throw a bunch of other rocks over your flat ones, preferably ugly ones you don't mind burying. I don't want my pile of rocks inside to come up to the top of the wall so I'll use just a few inches of stones, topped off by the prettier ones I want to be seen. I'll even throw some big ones on top to be prominently displayed. And your retaining wall is complete.