When you are logged in to your account, go to the domain dashboard for which you want to add the record. Click manage DNS records.
In the Hostname field, if you are setting up the record for the root domain, e.g. mydomain.com, you can leave it blank. It's usually left blank, and once the record is saved the field will show @.
If you are setting up the record for a subdomain, e.g. mail.mydomain.com, then you would enter "mail" in that field.
Choose MX from the Type drop-down menu. This will cause the Priority field to appear. Enter the priority / preference number. 0 (zero) cannot be used. If your email hosting provider has instructed you to use 0, use 1 instead.
Enter the MX record DNS value, e.g. ASPMX.L.GOOGLE.COM or mx2.zoho.com, in the Value field. You will likely have instructions to add multiple MX records, and each one will have to be slightly different.
Leave the TTL field as is at 3600. Click the blue add button to add the MX record to the domain's zone. It will appear in the list of DNS records for your domain. Click the green Save DNS Settings button to save the new settings.
Many email hosting providers give instructions to add quite a few DNS records to set up email. We have one-click installs for many of these providers, like G Suite, Zoho, or FastMail in our Marketplace so you don't have to type everything in manually.