Open source software gives a solo or small law firm rare control over its cost, its client data, and its exit options, but the same control transfers duties a vendor would otherwise carry. This guide builds a realistic open source stack layer by layer, from a Linux desktop with LUKS full-disk encryption to a self-hosted Nextcloud collaboration server, and maps each choice onto the professional-responsibility rules that now govern technology decisions: the duty of technology competence under ABA Model Rule 1.1, Comment 8; the duty to make reasonable efforts to safeguard client information under Rule 1.6(c); and the duty to supervise nonlawyer vendors under Rules 5.1 and 5.3. It explains how the permissive and copyleft license families actually work, why enforcement cases like Jacobsen v. Katzer matter, and how open data formats hedge against vendor lock-in and the Rule 1.16(d) duty to return client files. A phased migration plan, an honest total-cost-of-ownership analysis, and a worked hypothetical show how a firm can execute the switch deliberately and defensibly. The companion resource on the business and ethics case rounds out the philosophy; this is the version you can start on this quarter.