With the caveat that I did this back in 2011-2012: My own posts on installing Koha.
A couple links to pages that made the migration far easier:
VOKAL users guide: The Vermont Organisation of Koha Automated Libraries wrote a guide to help their libraries migrate to Koha, and it is super helpful, not least because a bunch of the libraries were using Follett software before they migrated, and Athena is Follett software.
Koha Mailing List Archives: there’s a good chance someone else has the question you have and it’s been answered on the Koha mailing list.
NExpress Tutorials: The North East Kansas Library System has a bunch of useful tutorials.
A Koha Diary: Arcadia Public Library System migrates to Koha and blogs all about it.