The way I did it, you don't have to remove the side panel (although it's easier if you do). But you will need a funnel and tube to reach the radiator from the front of the gas tank.
- Remove the radiator cap by reaching through the front of the bike.
- Slowly fill the radiator until it reaches the level of the cap.
- A small tube connects the top of the radiator with the overflow bottle. Slowly add coolant until the bottle is full.
- Basically, you fill a little, check the level and fill again.
- Put the radiator cap back on.
As you ride, air from inside the radiator will be pushed out the overflow and coolant will get pulled into the radiator. Add coolant every few rides until the level stabilizes.
And make sure your coolant hoses are secure and not leaking. I found a small leak in the right side main hose where it connects to the engine after about 2500 miles.