To write a check for cash, write "Cash" on the "Pay to the order of" line instead of a person's or business's name. Fill in the rest of the check — date, amount, signature — as usual.
Why this is riskier than a normal check
A check made out to "Cash" can be cashed by whoever is holding it — it doesn't require endorsement by a specific named payee the way a normal check does. If it's lost or stolen before you hand it over, anyone can cash it.
Because of that, only write a check this way when you're handing it directly to the person who will cash it, and avoid mailing a check made out to cash.