Once you have created both a quote and an opportunity it's super easy to link them together. It also does not matter if you are viewing the quote or the opportunity, you can link them together from either.

Quote

In the overview tab, just open the drop-down list next to Opportunity and it will show which current opportunities are open for the customer/prospect.

Opportunity

If you are viewing the opportunity, select the tab 'Quote'. Click the search bar to get suggestions of existing quotes or search manually by typing the quote name or number.

Select the relevant quote and click save. The opportunity has now been updated with the value of the quote 💰