Key steps to deploy Office 365 in a hybrid environment

Source: https://community.office365.com/modg/default.aspx 
Compiled and summarized by: Miha Kralj

• Develop a project plan: Your deployment project plan is used to schedule and track Office 365 deployment workstream progress. The plan should include a migration schedule that shows when each user’s mailbox will be migrated to Office 365. A high-level planning template is available in Appendix B: Deployment Planning Template for organizations to use for deployment project planning.
• Obtain hardware: You organization must procure the necessary hardware for on-premises components of Office 365 —such as the hardware for a hybrid deployment and identity federation servers.
• Evaluate network: Ensure your on-premises network meets bandwidth requirement and implement changes or upgrades as needed.
• Acquire SSL certificates: Third-party SSL certificates must be obtained and installed in your infrastructure to provide enterprise-security for Office 365 service offerings.
• Install and configure Active Directory Federation Services 2.0: To enable single sign-on, your organization must setup Active Directory Federation Services (AD FS) 2.0 federation and proxy servers prior to the migration of the first production mailbox.
• Install and configure Exchange hybrid servers: For the Exchange hybrid deployment scenario in which some of your Exchange users have mailboxes on-premises and other mailboxes on Exchange Online, you must set up an Exchange 2010 hybrid server prior to the migration of the first production mailbox.
• Install and configure the Directory Synchronization Tool: In an Exchange hybrid deployment scenario, the Microsoft Online Services Directory Synchronization Tool (DirSync) is required to keep your local Active Directory environment synchronized with your Microsoft Office 365 directory. The synchronization tool must be installed prior to migration of first production mailbox.
• Ensure that each end user workstation meets Office 365 client prerequisites: Each end user PC must meet the Office 365 client requirements.
• Configure Outlook Anywhere: You must configure Outlook clients to connect to Exchange Online over the Internet using the Outlook Anywhere connectivity (remote procedure call over HTTP) feature.
• Plan and coordinate all end user support: Before migration of first production mailboxes, you must plan for support of Office 365 users. This includes planning for escalation of support issues to Microsoft via a maximum of two points of contact (for example, the customer’s project manager and technical lead or Exchange Administrator).
• Plan, develop and execute training plans: You will need to implement training for your Office 365 end users.
• Plan, develop and execute communication plans: You will need to inform all end users about the transition to Office 365 service offerings and what is required of them.
• Perform migration of client-side data: Moving mailbox data and other client content to the Office 365 environment is the responsibility of the customer organization.
• Assign resources to initiate, monitor, and complete migrations: During migration windows, personnel in your organization must be available to manage migration activities, which often occur on evenings and weekends.