Using a standardized security configuration guide specifically designed for the intended system or platform is the best method to establish a secure baseline. These guides, often developed by experts and incorporating industry best practices, ensure that all devices start from a common, secure state before being introduced into the production environment. They typically include settings and configurations that have been widely recognized and vetted for their effectiveness in securing devices. While patching, encryption, and decommissioning play roles in the lifecycle of maintaining security for active devices, they are not methods used for establishing the baseline itself.