If all you want to do is enter the final score, you can easily do that through Game Results & Documentation. However, if you need to enter player statistics or scores for portions of the game, you need to do that from within the fixture.
Hover over Competitions and select Manage Competitions.
Select your competition. Go to the Fixtures/Results tab.
Click on the Enter Results icon.
Enter the period scores and the final score.
Scroll to the bottom and click Save.
You get a confirmation box. Click OK.
Your scores are now saved.