Attach your computer sensor to the frame so you can get a reading from the chainrings instead of the wheel, and put 4 or 5 magnets on the arms of the chainring. Your computer will then give a cadence readout (not the actual cadence number but in proportion). Watch your cadence while riding and shift gears to keep it between 80 and 87. Pedal continuously without concern for speed. When you get accustomed to this cadence increase the pace a notch, eventually working up to maintaining 90-95.