Once you sign in, select "Create a project..." from the "Select a project" menu near the top of the page.
Enter anything you'd like for the Project name, then click the "Create" button.
After the project is created (it will take a few seconds) you should be on the "Library" page for your new project.
Double-check to make sure that your new project name is visible from the menu at the top of the page.
Under the "Google APIs" tab, click the link for "YouTube Data API".
Click the "Enable API" button.
After the API is enabled (it will take a few seconds), click the "Credentials" link on the left side of the page.
Click the blue "Create credentials" button.
In the dropdown that appears, click the link for "API key".
Copy your new API key.
On the TubePress options page, click the "Feed" tab.
Enter your API key into the text field for "Google API Key", as shown below.
Click the blue "Save" button at the bottom of the TubePress options page. That's it!