To link accounts:
Note: In order to link accounts you need:
Analytics Edit permission.
Google Ads Administrative access.
Start Google Analytics.
Click Admin and choose which account and property you’d like to manage.
Under property, click Google Ads linking.
Click + New Link Group.
Select the accounts you want to link.
Enter your Link Group Title.
Select all sites where you want Google Ads data.
Click link accounts.
Extract from my Book ‘Making Google Ads Work’.
Read it FREE online (blog posts)