Collect a solar cell, desk lamp, multimeter, connecting leads and a piece of dark paper big enough to completely cover the solar cell.
This is best performed in a darkened room. If done in ambient light conditions ensure the light levels are constant.
Measure the area of the solar cell.
Connect the solar cell to the multimeter and set up the lamp so the beam covers the surface entirely. Mark positions and try not to move the lamp or cell again.
Cover the solar cell with the paper and record the voltage.
Uncover a measured area of solar cell (choice of value will depend on your solar cell size) - you should aim to have around 1/10th showing.
Record the value on the multimeter, uncover another 1/10th of the area and son on until the entire surface is exposed to the light.
For best results repeat your experiment 3 times.