To sync between Basecamp and Google Sheets, Syncopotamus needs to have permission to the accounts.

After creating your Syncopotamus account, you should see the Integrations page with both accounts saying not connected:

You can get to the Integrations page from any other page by clicking the menu button...

And selecting Integrations...

To connect your Basecamp account, click the Connect Basecamp account button, and select Yes, I'll allow access

You should be directed back to the Integrations page. But now your Basecamp information should be visible

Now all that's left is to connect your Google account.
Click the Connect Google account button and you will be directed to Google's sign in page.

Select the account you want to use with Syncopotamus, make sure this account has access to the Google Sheets you want to synchronize.

Click Select all to give Syncopotamus the permissions needed (more info about permissions)

Once you've clicked continue, you should be redirected back to the Integrations page, and now your Google name and email should be visible:

Congratulations! Now you're ready to start synchronizing.