If the linked tutorial is outdated, follow this steps:

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Go to https://console.developers.google.com/

Click the project selector:

Click an existing project, or click NEW PROJECT:

Give it a name, click CREATE:

Wait a few seconds until the project is created, make sure the new project is the current selected one. Then click ENABLE APIS AND SERVICES:

Now let's enable Google Drive and Google Sheets.

Search for "drive" ...

Click on the "Google Drive API" item

Click on ENABLE:

Do the same for Google Sheets now

Search for "sheet" ...

Click on ENABLE:

Click on the hamburger menu:

Inside the menu, open APIs & Services ›, and click Credentials:

Inside Credentials section, click on + CREATE CREDENTIALS:

Select the Service account from the dropdown:

Inside Service account details, you just need to provide some account name. step (1): I will use "fts-flutter", then click CREATE AND CONTINUE:

step (2): No need to do anything else, click CONTINUE

step (3): No need to do anything else, click DONE

You will see the new item, under the Service Accounts section, click the pen icon to Edit service account:

On the tab bar, click on KEYS:

On the Keys section, click on the ADD KEY ▼ dropdown, and Create new key:

Select "Key type" > 🔘 JSON and click CREATE

Congratulations! You finally have your service credentials to use on any fts project.

Ideally, rename the file to credentials.json and put it in some global folder in your computer. You can reuse it on many FTS projects, placing the absolute path to it.