Create one safe space for a change to exist before you touch a slider or start painting on an image. That is an easy way of looking at layers. Layers allow an adjustment to exist so that you can try an edit without it becoming a part of the original. This is very important for artistic retouching because an adjustment may look good for the first few minutes of testing but look bad by a couple minutes of comparison.
It is helpful to imagine the original image as the layer that you never touch. You can place a retouching layer, a separate adjustment layer, or a layer mask on top of your original that will allow you to control where a particular change occurs. That setup allows for non-destructive editing, where the original is never changed. It may look like it needs an adjustment and it does, but the skin appears too smooth, the shadow details look lost or crushed, or the colors are oversaturated. You can easily reduce the strength of the edit, soften the mask edge, or temporarily remove the layer and see the original image.
Start by making only one kind of edit per layer for your first layer exercise. This might be a single layer for minor blemish correction, another layer for a gentle tonal change, and yet another layer for a color adjustment. You might think that adding names to each one is overkill to start, but labeling each layer will give you an easy way to see what each layer is doing. Without it, it is easy to look at an image where the original has been heavily edited on the same layer and ask what caused a particular visual element to occur. Was the blemish removal done with the clone stamp, the curves adjusted to lighten shadows, the saturation of the colors boosted, or was there a local brush over the image? This is the benefit of separating adjustments into their own layers: you can see the results of individual edits to see if you think they are appropriate for the overall image.
A layer mask can also come in handy when you want to make local edits. If a cheek or background area is dark, you may be tempted to lift up the entire image with the Brightness adjustment, but using a layer mask allows you to adjust only a specific area. If you start by painting on the mask with a low opacity setting and using a soft brush edge, you can build up the effect in that area to see how it looks before you paint the edges. This way, if the edge of the mask on a hair or edge of clothing is hard-looking, that tells you that you need to soften the mask more. If the edge of the adjustment is hard, the edit will stick out rather than blend in.
The hardest thing to learn when using layers for artistic retouching is to appreciate the smallest edits. The biggest, strongest adjustments are very noticeable because they are so easy to see. But artistic retouching works best when the smallest, quietest adjustments support the image, rather than the brightest and loudest changes. This is another good reason to use layers. If you are unsure about a strong contrast or color adjustment, try turning the layer on and off to see if you like the look with or without it. You can also try setting that layer at 50 percent to start. Looking at the whole image, including details like skin texture and hair, and seeing the effects without the whole image looking overly processed can help you see if a heavy-handed edit is hurting the photo instead of helping it. The strongest version is not always the best version.
Another reason to use layers is to give yourself room to make a change without fear of not being able to take it back. It can be frustrating to see a pattern of skin texture repeating from using the clone stamp, but that problem can be erased if you used a layer for the spot correction. If the shadows get crushed by a heavy adjustment using curves, you can use a layer opacity to lift the shadows back up. If highlights get burned by a burn pass in a local adjustment, the effect can also be adjusted to a more natural look using layer opacity. That way, you can try different things to achieve the right look and texture, tone, and final sharpness without being scared that you will not be able to take it back.
The next time you want to do some artistic retouching on a photograph, see if you can try using a couple of fewer edits, but spread them across more layers. Maybe one layer for retouching, a single adjustment layer for toning, and one controlled adjustment with a layer mask for local correction and a final layer adjustment for color and sharpness. As a final exercise, if you can find a particular layer and describe exactly what it is doing to the image, that can help you see the edit, and correct it, and keep the image looking more natural.