Same as for any other widgets - the easiest way is probably to long-press on an empty bit of the desktop screen, choose 'Widgets' from the resulting menu, and then select one of the two AquaMail options from the list: 1x1 for a single icon indicating number of messages, or 4x2 for a scrollable message window inside a re-sizeable widget. It depends on the version of Android how you put them on the desktop, sometimes just a click, sometimes press-and-hold and then drag into a spare space. You can then customise the final look.
Andre