Change the content of styles/prosilver/template/imprint.html to your needs
Copy files into phpBB/ext/nachtelb/imprint
Go to "ACP" > "Customise" > "Extensions" and enable the "Nachtelb/Imprint" extension.
The Position of the link to the impress-page could be changed. You have to change the filename of nachtelb/imprint/styles/prosilver/template/event/overall_footer_copyright_append.html to the event-name of another position in the template. You could find those event-names in the phpbb-dev-wiki.
Please use the "Issues"-Function from Github.