Google homepage
To add OnOneMap to your Google personlised homepage, first visit your google homepage by going to the main google site, and if necessary click "Personalised Home" or "Personlise This Page" to the top right of the screen (you will need to do this if you last viewed google in the "Classic" mode - i.e. not personalised):
If you have not previously used the personalised homepage, you should now click "Make it your own" beneath the search bar and then the "Show my page" button on the far right.
Click "Add stuff" to the right of the page, about a fifth of the way down:

Click "Add by URL" which appears to the right of the seach bar:

You will see the following dialog:
Enter the URL "http://ononemap.com/widgets/google" and then click "Add"; you will see an alert box warning you that the feature you are adding was not created by google. Click "OK".
Click "Back to homepage" at the top left of the screen to see the widget in action.