First, head over to https://app.datatrics.com/channel and search for Google Analytics.
Click on the green "Integrate" button and you will be redirected to the following screen:
Click on "Connect to Google Analytics".
Log in with an account that has permission to edit in the Google Analytics account you want to connect.
Allow Datatrics to access the data.
In the screen above, you can select the account you want to connect to your project.
After selecting and saving, you can also allow Datatrics to manage you custom dimensions. This is for reporting and sharing custom audiences, also with Google Ads.
If you want to set up your custom dimensions manually, you can find the explanation over here.