To update your nameservers at Google Domains and have your domain point to Skystra, please follow the below steps.
1. Login to your Google Domains account.
2. Select the domain name you wish to edit.
3. In the top left, click the Menu button and select the DNS option.
4. Under the Name servers section, select the Use custom name servers option.
5. Input Skystra nameservers into the Name server fields (click + to add the second nameserver):
6. Click the Save button to save changes.
It can take up to 24 hours for the DNS changes to propagate (DNS propagation is the process of updating every server across the internet with new information) globally.
You can use whatsmydns to track the progress of the DNS propagation for your domain.