Custom instructions are the cheapest upgrade ChatGPT has. They cost nothing, they apply to every new chat, and most people either leave them blank or fill them with vague wishes like "be helpful." You will find them under Settings, then Personalization.
That screen now has three layers. There is a personality preset under "Base style and tone," with options such as Default, Professional, Friendly, Candid, Quirky, Efficient, Nerdy and Cynical. There is a free text box for your own instructions. And there is a section about you, where you can give your name, your job and some context. The preset sets the mood. The instructions are where the real control is.
Start by telling it who you are in one plain sentence. Something like "I am a freelance bookkeeper in Ohio and most of my questions are about small business taxes and spreadsheets." That single line stops it from explaining basics you already know and from assuming you live somewhere else.
Next, ban the padding. A sentence such as "Skip the introduction and the summary, answer first, and do not repeat my question back to me" removes a surprising amount of filler. If you want shorter answers, say how short: "Keep answers under 150 words unless I ask for detail" works better than "be concise."
Then tell it how to handle uncertainty, which is the instruction that matters most. Try "If you are not sure of a fact, say so plainly, and never invent a source, a quote or a statistic." It will not make the model perfect, but it makes the model more likely to flag its own guesses.
Add a line about format if lists annoy you. "Write in short paragraphs and only use a bullet list when I ask for one" changes the feel of every reply. If you write in American English, say that too, because it will otherwise drift.
Finally, add one instruction about pushback, such as "If my plan has an obvious flaw, point it out before helping me with it." That turns a people pleaser into something closer to a colleague.
Keep the whole thing short. Every sentence competes for attention, and a long list of rules gets followed less reliably than five clear ones.
The short version: write one sentence about who you are, one against padding, one about uncertainty, one about format and one inviting pushback. Save it, start a new chat, and adjust after a week.
