Unlock the club kiosk screen and choose Courts on the main menu. You should then see today's court usage from the current time onward. The current or next immediate court time slots will show a yellow Light button. Click this and you'll be asked to confirm to buy more light, you can choose the amount of light that you'd like in certain increments.
The light will stay on until the end of the chosen increment so if you finish before this time make sure you unlock the screen again and choose to switch it back off. You'll be informed about the cost at this point which will be a pro-rata charge for the total time used.