First, head over to your channel list and search for "sendinblue"
Click on the green "integrate" button and the following page will open up:
As you can see, the only thing you need to fill in is an API Key.
Fortunately Sendinblue has their own documentation showing you exactly how to retrieve this key depending on your version. Check out this link if you need any help.
Note: We support both V2 as well as V3. We do however recommend you use V3 because it's the most up to date (and supported by sendinblue) version.