SPF
Add include:zoho.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.
A complete record with Zoho Mail alone looks like v=spf1 include:zoho.com ~all. If you use other sending tools, their includes go in the same record.
DKIM
Found in Zoho Mail Admin Console, under Domains, then the domain, then Email Configuration.
Zoho lets you pick the selector name, which means you should write down what you chose. An unusual selector is invisible to any external check, including ours.
Zoho Mail publishes keys under the selector zoho or a custom selector you choose, 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 Zoho Mail
Zoho's free tier is common among very small businesses, and those accounts are often set up once and never revisited. Check the record still matches what the console shows.
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.