SPF
Add include:spf.mtasv.net 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 Postmark account, under Sender Signatures or Domains.
Postmark gives you a DKIM TXT record to publish, and verifies it before allowing sending. It also supports a Return-Path CNAME, which improves alignment.
Postmark publishes keys under a selector Postmark provides, commonly pm, 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 Postmark
Postmark is transactional only and refuses bulk marketing mail, which is why its deliverability is unusually good. If you send both kinds, keep them on separate subdomains with separate records.
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.