STEP 1 - Change domain nameservers
Hostinger's nameservers are:
If your domain is purchased at Hostinger, you can set the default nameservers via the domain management page.
STEP 2 - Add domain to your hosting plan
The next step is to add the domain to your hosting plan. You can do this by:
Adding domain as an add-on to an already active hosting account at Hosting → Add Website:
Or Hosting → Manage → Add Website:
Your website might be unreachable during the DNS changes propagation time, which can take up to 24 hours.
Feel free to check the article on how to transfer the domain to Hostinger.