The Lasso Selection tool will let you draw out your own custom selection.
Instead of clicking and dragging as you would with the others, you will draw the outline of the area you want to select.
To add a new area to your selection, press and hold SHIFT before you draw out a new selection.
To subtract an area from your selection press and hold Alt before you draw.
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 in Magma 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.