Click on the thumbnail of a layer to edit that layer--an Image menu will appear on the left.
To delete a layer, click on that layer, then click the trashcan icon in the small layer menu that appears next to the selected layer.
A zoom percentage dropdown appears in another menu above the image, so you can zoom in for more precise drawing or erasing.
Move the thumbnails up and down to bring layers forward or back in the image.
Move a layer you want to edit to the top to see it as you work, then move it to its final position.
A temporary layer of a solid-colored rectangle from Elements - Shapes can be very handy just behind a layer with no background.
You can reduce the Opacity of any layer. This can be temporary to help you position different layers together, or you can make a layer partly transparent as a special effect.
Remove object and Insert object cannot be used after Remove background or Erase on the same layer--Adobe Express will ask if you want to cancel the new command or discard the earlier edits.
Remove object and Insert object let you choose a brush size to highlight the part of the selected layer you'd like to change.
Remove object takes out something, and the AI fills in that area with an attempt to show what was behind the item.
Insert object has you type a description of what you want to see in the highlighted area.
Both will show you several options to let you choose your favorite. Click Keep when you are happy with the current option, then Close.
Remove background will make the AI take out everything it perceives as background within the layer.
Use this tool on all your layers except the one at the very back of your image.
The removed sections will be transparent, and the layer outline may shrink down to only fit what is left.
If Remove background removes something you wanted to keep, undo the change (as with Ctrl-Z) and try Erase instead.
Erase is very useful for cleaning up a layer after using Remove background.
Erase gives you a choice of brush size and two brush types:
In either case, it's useful to work in small steps and click Done frequently. That way, it's easy to undo your more recent change without losing earlier edits you want to keep.
When you click on a layer directly or through its thumbnail, a thin blue border will appear around it.
Other related options appear in the Image menu to the left.
There are three choices in the Image menu for editing the colors within a layer.
Adjust is the most useful, because it lets you adjust the Contrast and Brightness of the layer. This can draw attention to the layer containing the main subject of an illustration, or help another layer blend into the shadows. Saturation and Warmth can be fun to experiment with as well - for example, lower Warmth makes a scene look more like a nighttime setting or cloudy day.
Opacity changes how solid or transparent the layer is, which can serve as a color effect.
Effects (in the tab above the main Edit options) can be useful if you want to make everything in the layer darker, tinted, or grayscale, with some Duotone options that can be interesting.