How do I make my repayment?

You can make the deferral payments through the Electronic Federal Tax Payment System (EFTPS), by credit or debit card, or with a check or money ord.... Note: you must make these payments separate from other tax payments to ensure they're applied to the deferred payroll tax balance. IRS systems won't recognize the payment if it's with other tax payments or sent as a deposit.

https://www.irs.gov/individuals/understanding-your-cp256v-notice