Hand-tools only: saw off larger piece with a fine-toothed saw, using a jig or in a mitre box, then plane down using a suitable low-rise planing stop (in a jig or just on the benchtop) or using a planing cradle. If you'll be doing a lot it's worth building the cradle, both for consistency and speed since you can work on longer strips with ease. These are then crosscut to produce individual keys.
Power tools only: saw off larger piece using a bandsaw and then refine on a belt sander?
But you could just as easily combine approaches, saw on the bandsaw and then plane to thickness, some would argue this is the best approach overall but hand sawing such small pieces isn't slow or difficult.