Which one, actually, you ask? How to restore a wallet or how to restore someone's wallet?

Because if you are asking for a procedure to restore a wallet, we have some guides here, but it depends on what wallet. Is that Electrum, Bitcoin Core, or any other wallet?
Adding more details can help us to know if that wallet is recoverable or not.

If that's other people's wallet, you should have some experience before offering a recovery service.