How do I migrate to Microsoft 365?

The shape of it, for a typical small business:

• Before anything moves: we confirm who owns your domain, decide licensing, and inventory what is actually in the old system. Shared mailboxes, distribution lists, calendars, and aliases are what get forgotten and then cause complaints in week two.
• The bulk copy: mail, calendars, and contacts copy across while everyone carries on working in the old system. Depending on volume this takes hours to a few days, and nobody notices it happening.
• Cutover: we change where your mail is delivered, usually in the evening. A final catch-up sync brings across anything that arrived during the copy.
• The next morning: Outlook reconfigured, phones reconnected, and somebody available for the inevitable three people whose setup is unusual.

Two things that decide how smooth it is. First, whether you control your own domain, because if it is registered to a former web designer or an old employee's personal account, that becomes step zero. Second, how much mail there is. Fifteen years of a 40 GB mailbox is a different job from a tidy 3 GB one.

Back up your new tenant from day one. Microsoft keeps your service running, it does not keep your data safe from deletion or ransomware.