Sometimes you only want to make changes to a clearly defined area of your layer. This is where your selection tools come in. This first one is a rectangle-shaped selection and the keyboard shortcut for this is M.
Click on your canvas and drag to create a rectangle selection of a portion of the layer you are currently active on.
Holding SHIFT while you drag will create a perfect square selection.
Pressing Alt while dragging will center your selection around the point that you clicked.
To add a new area to your selection, press and hold SHIFT before you click and drag.
To subtract an area from your selection press and hold ALT before you click and drag.
You can move your selection by clicking inside the selected area and dragging it to your preferred position. This doesn't move the contents of the selection.
To deselect, simply tap or click outside the selected area.
Your settings toolbar beneath your color wheel gives you a few options for your selection: Select All, Deselect, or Invert Selection.
The shortcut keys for these options are:
Select all: Ctrl+A
Invert Selection: Ctrl+Shift+I
You can also erase the contents of your selection by pressing Delete.
Once you have an area selected, you can adjust the size of its contents by switching to the Transform tool.
You can also paint or erase within your selected area giving you a clean edge.
You can set something that you have drawn as your Magma avatar using the selection tools. Make your selection and then click/tap the Set as my avatar button in the Tool Settings panel.