When you open a photo in Photoshop, it is opened in a layer – shown in the Layers Panel on the right hand side as Background. When you edit a photo, it can be useful to put things into different layers, to isolate them from the rest of the picture. You can then apply effects or make changes in one layer only. This is a very basic description of how to create layers.
I like to make a duplicate layer of the whole photo before I do anything else; that way, I don’t change the original in the Background layer, so I can always go back to it for comparison. To do this, click on Layer, then click on Duplicate Layer. A dialog box opens, with a suggested name for the layer as “Background copy” – you can use this name or change it. Click OK.
You now see two layers in the Layers Panel.
Choose a feature in the photo, for example someone’s face; select this area using the Magnetic Lasso Tool, or one of the other tools. Now do Layer, then New, then Layer via copy. Click OK in the dialog box.
You will now see a third layer in the Layers Panel.
If you click on the eye symbol next to the Background layer, and the Background copy layer that you made, then both these layers are no longer visible, and you are left with just the newest layer containing the face (or whatever feature you chose). In the Layers Panel, click on that layer – this is now the layer you are working in. You can make any changes you like in this layer, without affecting the rest of the photo.
To see how your changes will look, turn the Background Copy layer back on by clicking on the eye symbol to make it visible again.
It is important to be aware of which layer is the current layer – when you make changes they will only happen in the current layer – so remember to click in the correct layer in the Layers Panel before you make your changes!
When you save your picture, if you wish to keep the layer structure, you need to save it as a .psd file; if you save it as a jpeg, the layer structure will not be there when you next edit it.