Shifting gears
I don't recommend shifting gears during a wheelie unless you are good at wheelies, and are able to use the clutch in the process. Otherwise, shifting during wheelies is hard on the transmission. It is also hard on the fork seals if you miss a shift. My advice is to learn to ride wheelies at a constant speed. Then there will be no need to shift.
How to set a wheelie down
When bringing down a wheelie, stay on the throttle until the front end is safely on the ground. If it is necessary to quickly bring down the front end, then close the throttle at first. Then as the front is coming down, open the throttle. In that way you will have a soft landing.
Step by step procedure to launch a wheelie for a beginner
1. Drop the tire pressure to about 15-20psi
2. Put the bike into first gear
3. Go about 15mph
4. Pull in the clutch
5. Rev up the engine a little and drop the clutch
6. Repeat step 5, increasing the rpm's, until the front end comes up close to the balance point.
7. Reduce the throttle as the front end comes up to the balance point.
8. Cover the rear brake.
9. Stay on the throttle as it comes back down.