I would practice in sand if possible. If you don't have sand, grass is a decent second option. Falling off the back is a right of passage when learning. It eventually happens given enough time so practice at LOW speed. I would not try anything beyond 2nd gear wheelies on that 250R until you have a lot of practice time under your belt.
With enough practice, you will be able to ride a wheelie for quarter mile using any of the techniques you're learned.
Wanted:
1985 200X OEM Airbox Thin Vacuum Line
1985 200X OEM Swing Arm Skid Plate