/openils/var/templates/marc/k_book.xml
By looking at k_book.xml you'll see the file structure needed for the template. However, the tricky part is managing the spacing for your fixed field data.
One method is to locate an existing record in the database that already has the desired MARC structure, and use this for your template.
SELECT marc from biblio.record_entry where tcn_value='ABC123';
marc
----------------------------------------------------
01571cam a2200409 a4500
20090228144931.0
920728 000 0
9789042023703
Birken, Marcia
Discovering patterns in mathematics and poetry
ILL
University of Oklahoma Libraries
Copy/Paste this XML into a new file, clear out the data specific to this record, and clean up any unwanted tag data. For instance, all that's needed is