It’s an ongoing process

Planning your data strategy begins well before you launch a migration project and continues long after you successfully move your data over to SharePoint Online. It’s an ongoing process and one that requires you to plan and manage policies and processes.
Taking the time to prepare will help you determine the best ways to gather the content, organize the chaos, and bring data to your newly organized SharePoint structure.
Summing up key best practices:

Assess third party compatibility and product dependencies.

Identify data sources.

Analyze what needs to be moved: types of content, volume, and retrieval patterns.

Define the new system requirements – format, metadata and policies.

Do not waste resources migrating content that fails to meet business needs.

Conduct trial runs to determine the scope and time-frame of the project.

Post migration, confirm that your SharePoint sites and content meet all of the standards defined in your governance plan.