Well, you need some kind of ballast resistor or constant current source like LEDs almost always do. Then just an N-channel MOSFET between that and ground. Assuming the LED is 7V, 700 mA then something like the pic attached.
If you need higher efficiency then you'll want to use a switching constant current driver. That only really needed for battery powered stuff https://social.umeahackerspace.se/attachment/523383