You can't have had a Nationwide switch bonus since 2021, unless it was for a sole account & you now switch to a joint account or vice versa.

Ensure the account you switch from has at least two direct debits (Switch Tracker can help with this)

Open a new Nationwide FlexDirect account

Switch in an account within 28 days of opening

Pay in £1,000 or more

Make at least one debit card payment

Wait up to 10 days of meeting all criteria to receive the £175