SPF
Add include:_spf.protonmail.ch 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.
A complete record with Proton Mail alone looks like v=spf1 include:_spf.protonmail.ch ~all. If you use other sending tools, their includes go in the same record.
DKIM
Found in Settings, then Domain names, then the domain.
Proton provides three CNAME records covering key rotation. All three should be published, and Proton verifies them before enabling signing.
Proton Mail publishes keys under the selector protonmail, protonmail2, protonmail3, 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 for a few weeks, authorise everything legitimate, and only then tighten. The DMARC guide covers the order in detail, and the order is what stops you blocking your own invoices.
Worth knowing with Proton Mail
Proton requires domain verification before mail flows at all, so a partial setup usually shows as mail not arriving rather than as an authentication warning.
Check it afterwards
Publishing the records is not the same as them working. Verify from outside: whether SPF passes, whether a DKIM signature is actually present, and how many lookups your SPF now performs. The free mail check answers all three in plain language.