The source code of landmark software developed since the beginning of the computer era is a precious part of our cultural heritage, and needs to be properly rescued, curated, archived and made available to present and future generations. In this article, we present the Software Heritage Acquisition Process, that has been designed to provide detailed guidelines on how to perform this important task, preserving important historical information. This process has been validated extensively on several important pieces of software source code of historical relevance in the University of Pisa, in collaboration with UNESCO, and is open to all for adoption and improvement.