SPF
Add include:_spf.klaviyo.com to your existing SPF record. Do not create a second record: two SPF records mean neither works, and that is the most common mistake in this whole area.
DKIM
Found in the Klaviyo account, under Settings, then Domains.
Klaviyo uses CNAME records for both DKIM and the sending domain, which means Klaviyo manages rotation.
Klaviyo publishes keys under Klaviyo-provided selectors, which is what an external check looks for.
DMARC
DMARC is the same regardless of provider, because it is your instruction rather than theirs. Start at v=DMARC1; p=none; rua=mailto:you@yourdomain.com, read the reports, authorise everything legitimate, and only then tighten. The DMARC guide covers the order, and the order is what stops you blocking your own invoices.
Worth knowing with Klaviyo
Klaviyo is usually set up by a marketing team rather than whoever manages DNS, so the records get added once by someone who then leaves. Worth confirming they still match what the account expects.
Watch the lookup count
Every include costs one of your ten SPF lookups, and so does every include inside it. Adding a third or fourth sender is where domains typically cross the limit, after which the whole record fails silently. See the lookup guide before adding another.