You need to download and install an application called Rainmeter. Then head over to killall-q's deviantart gallery and grab his skin: GW2 WvW Overlay.
If you installed rainmeter, when you click to install the skin it will automatically activate it. Choose your world server, move the overlay to where it suits you and you are good to go.
Important notes regarding the overlay:
GW2's display resolution must be set to "Windowed Fullscreen" or "Window".
• Click scores for map select menu.
• Double-click scores to toggle minimize.
• Click objectives to copy waypoints/points of interest and RI times.
• Middle-click objectives to set siege decay timers.
• Scroll to zoom.
• Access world select in right-click menu.
• After weekly WvW reset, right-click menu ➨ "Refresh skin" to retrieve the new match up.