I learned to use the Interface the easy way by getting pointers from BasetradeTV's Rifkin, but other people might struggle getting things to show up correctly.
Here's some pointers to bootstrap your experience.
Installing the UI
1. Download the UI
2. Save it in the Interfaces folder in \Users\YourWindowsUserName\Documents\StarCraft II\Interfaces
If you have an old version of GameHeart in this folder, you're better off deleting the old version (because of ingame naming conflicts).
3. (Re)Start SC2, Select the interface in Menu -> Options -> Gameplay -> Observer Interface (GameHeart 1vs1)
4. Remember to switch it back to normal before you start observing teamgames instead of 1v1
After the game has loaded
1. CTRL+SHIFT+S, to bring out the Score
2. If the series is not a Bo3, hit CTRL+SHIFT+5/7/9
3. Line up the positions of the playernames to where they spawn on the map, toggle the positions in the interface by hitting CTRL+X (hotkey defined as Global->Observer->Toggle Versus Mode Sides)
4. Set the score, SHIFT+1/2/3 for the first player in the interface, CTRL+1/2/3 for the second player's score.
5. If the names/clantag combination is too long to properly show the player's name, press CTRL+SHIFT+C to hide the Clantags
6. Don't forget to pull out the Production Tab! (hotkey D)
7. Observe your (Game)Heart out!
Important Bonus tip: Learn to use ESC a lot to get the most out of the interface (will cancel selection and show resources lost, income, apm, etc.).
The rest of the hotkeys can be seen by pressing F11, you'll see this:
For general observer tips, see my previous blogpost.