Define it by looking backward rather than by choosing aspirationally. Collect the ten best things you have written: emails that landed well, an explanation a customer thanked you for, a proposal that won. Read them together and identify what they have in common. That is your voice already, in evidence, and it will be more accurate and more sustainable than a set of adjectives selected because they sound appealing.
Write it down as decisions rather than as descriptions. Adjectives like friendly and professional are useless because everybody claims them and nobody can act on them. What is usable is specific: we explain the reasoning rather than issuing conclusions, we name the tradeoff rather than only the recommendation, we use short sentences, we do not use exclamation marks, we say I rather than we. Somebody else should be able to apply that list without asking follow up questions.
Include the negative version, which is frequently more instructive. The words you never use, the moves you avoid, the register you will not adopt. For a business built on directness, that might be no jargon, no artificial urgency, no describing yourself as passionate. A short list of what you are not is easier to check against than a description of what you are.
Test it against the awkward material rather than the marketing. Anybody can sound like themselves writing a homepage. What reveals whether a voice is real is a price increase, a delay you have to explain, or a declined request. If your voice collapses into corporate neutrality the moment the news is bad, it was a marketing layer rather than a character, and customers notice the switch precisely because it happens at the moments they are paying most attention.
Keep it to one page and revisit it once a year. Voice drifts as a business grows, particularly when more people start writing, and the document exists so that drift is a decision rather than an accident. It is also the single most useful thing to hand somebody who writes on your behalf, including an AI tool, since output that sounds like you requires a description of what that means.