You can set automatic printing to on/off in dslrBooth > Settings > Print Setup. With automatic printing on, dslrBooth will print automatically at the end of each session.
In addition to automatic printing, if you want to print an image, click 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.