What are the basic principles of archival practice? How do archivists do their work?
This Manual for Small Archives from the Archives Association of British Columbia has a short summary of what an archive is, the history of the archive, and provides many suggestions for creating, maintaining, and organizing an archive, while also acknowledging that it offers more ideas than most organizations can or will want to use with their own archives.
The Society of American Archivists outlines Standards & Best Practices and includes links to other resources used by museum archives and archives proper. Again, this guide offers more information than most small museums will want, but could help answer any questions your institution may have.
This guide on Creating Museum Archives from Purdue University explains how a non-archivist could found an archive in a museum setting while being realistic about the challenges (space constraints, etc) that museums can face. It also outlines ten steps for starting an archive.
The Historical Society of Pennsylvania outlines Advice for Museums, an overview of archival theory and practiced designed for those who without archival training. It covers archival theory, preservation, description, cataloging, and includes some additional resources.
The Historical Society of Pennsylvania also has a resource page for small archives, offering information on preservation, as well as archival basics and cataloging.
Looking for an all-in-one resource for starting your own archival program? Check out this guide from Yale University.