Operational security is not one dramatic precaution. It is a stack of small habits, each cheap, that together keep an account standing.
A fresh identity
Pick a username you have never used anywhere else. Reusing a handle from a clear-web account is how a name gets tied to a market account across platforms.
A unique password
Generate a long unique password and keep it in an encrypted local manager, not a browser autofill and not a cloud service. See passwords and the mnemonic.
Session hygiene
Close the browser after a session rather than leaving it open for days. A long-lived session is a longer window for something to go wrong.
Small balances
Money on a market is money exposed to whatever happens to the market. Keep the balance sized to the current order and withdraw the rest to a wallet you control.