If your domain is registered with SiteGround, you can access your DNS Zone from your Client Area → Services → Domains→ Manage.
You can find a detailed guide on creating, editing, and deleting DNS records at SiteGround here: DNS Management.
To connect a domain with your Hostinger services, use the following values:
mx1.hostinger.com (Priority: 5)
mx2.hostinger.com (Priority: 10)
Hostinger A record - you can locate it in the Plan details section of your Members Area