Do it directly, in writing, with notice and a handover. It is much less dramatic than it feels beforehand.
What I send, roughly:
A clear statement with a date. I will be finishing our work together at the end of next month. Not thinking about, not may need to: a decision with a date.
A short, neutral reason. Changing focus, capacity, a shift in the kind of work I take. You do not owe a diagnosis of the relationship and giving one invites a negotiation about whether the diagnosis is fair.
What you will complete before then, specifically. This is what turns a resignation into a plan and it is what stops people panicking.
A handover offer. Documentation, a call with whoever takes over, access and credentials sorted. This is the part that determines what they say about you afterwards.
Notice length: for a year-long relationship, four to six weeks is normal and generous. For a project, finishing the current phase.
What to expect: some surprise, occasionally an offer of more money, occasionally a slightly cold fortnight. Almost never a fight. The version people fear: anger, damaged reputation - is rare, and it is much likelier if you leave badly than if you leave clearly.