The EOSC-hub project operates many of the core elements of the European Open Science Cloud, and acts as the main facilitator of onboarding new providers in EOSC through the EOSC Portal. This handbook provides practical information to providers about their responsibilities after they registered their services in the EOSC Portal. The document provides guidance on how to handle user access requests in the EOSC Marketplace, how to respond to EOSC user helpdesk tickets, how to react to and stay informed about security incidents, how to collect user feedback, where to raise issues about EOSC elements to other stakeholders, how to ensure customer-centric service delivery, and how to keep service profiles up-to-date in EOSC. The handbook aims to support service providers operating professionally in the EOSC context.