SPF
Add include:spf.smtp2go.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 SMTP2GO dashboard, under Sending, then Verified Senders.
SMTP2GO provides CNAME records for DKIM, which it then manages.
SMTP2GO publishes keys under an SMTP2GO-provided selector, 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 SMTP2GO
SMTP2GO is often chosen as a relay for applications and devices that cannot authenticate properly themselves. Those senders are the easiest to forget when auditing an SPF record.
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.