SPF
Add include:_spf.mlsend.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 MailerLite account, under Settings, then Domain authentication.
MailerLite provides SPF and DKIM records and verifies them before allowing authenticated sending.
MailerLite publishes keys under MailerLite-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 MailerLite
Newsletter platforms get trialled and abandoned more than most tools. If you tried MailerLite and moved on, the SPF include is probably still there.
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.