Tell arc which domain you own. We don't need access to your registrar — just four DNS records on the next step.
Add the records the dashboard shows when you add the domain, in this order. First the arcrelay-verify TXT (publish its value exactly as shown), SPF and DMARC. The dashboard shows the arcrelay-verify value only in the Add-domain dialog, so copy it before you click Verify & add or close the dialog; if you lose it, delete the domain and add it again to get a new value. Already have an SPF record? Add include:_spf.arc-relay.com to it; don't create a second one. Then click Verify (next step). Change MX to 10 mx.arc-relay.com only after the domain shows verified: the MX change routes all mail for the domain through arc, and arc refuses mail for a domain that isn't verified yet. Most apply within minutes; some take up to an hour.
Click Verify on the domain in your dashboard. Verify checks one record, the arcrelay-verify TXT at your bare domain, at the moment you click; nothing polls in the background. Not found yet? Give DNS a few minutes and click Verify again. Once it shows verified, switch your MX to 10 mx.arc-relay.com.
Create the first alias on your domain — say, hello@. Point it at the inbox you actually read.
Send an email from any account. It should land in your real inbox within a couple seconds.