You can set automatic printing to on/off in the preferences/settings. With automatic printing on, dslrBooth will print automatically at the end of each session.
In additional to automatic printing, if there’s an image that you want to print, click on the print icon. You can also use a keyboard shortcut. On windows, click CTRL-P or the print icon. On Mac, press Cmd-P. Every time you press CTRL-P, it'll print one copy.